Surficial geology of a part of the northeast flank of the Bridger Range, Montana
by Verne Keith Schrunk
A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE
in Earth Sciences
Montana State University
© Copyright by Verne Keith Schrunk (1976)
Abstract:
The surficial geology of a part of the northeast flank of the Bridger Range, Montana includes such
diverse deposits as till, in morainal form, colluvium, talus, landslide debris, and residuum.
Geomorphic evidence in the Fairy Creek basin suggests four distinct glacial advances and retreats. The
oldest advance is correlated with the Bull Lake Stage of the classical Rocky Mountain chronology as
based on the following criteria at this locality: 1. Till of this age is in a lower stratigraphic position than
the other tills here.
2. The moraine representing this advance is furthest downstream from the cirque.
3. The moraine has the most subdued surficial form.
4. There are fewer surface boulders on the moraine of this advance than on the other moraines.
The remaining three advances are correlated with stades I, II, and III of the Pinedale Stage of the
classical Rocky Mountain sequence.
As compared to the Bull Lake morainal deposits, Pinedale deposits (1) lie stratigraphically higher, (2)
are fresher in appearance, (3) have more boulders exposed at the surface, (4) are of lesser areal extent,
and (5) lie nearer to source cirques. Cache Creek and North Fork of Brackett Creek contain all these
deposits except for the Pinedale III stade. The South Fork of Brackett Creek, in contrast, displays the
three, stades of the Pinedale Stage but not the Bull Lake Stage.
Bedrock in the study area consists of sedimentary strata ranging in age from Late Devonian to Late
Cretaceous, though Triassic units are absent. Bedrock, where not covered by till, occurs as isolated
patches and knolls and in linear ridges and cliffs.
Land use considerations acknowledge a three-fold division of the study area. The western division is
most suitable for its scenic value and should not be developed. The central and eastern divisions could
be used for construction of all. types, but caution should be used in the eastern division because of
steep slopes and considerable quantities of bentonite present in the bedrock and surficial deposits. SURFICIAL GEOLOGY OF A PART OF THE NORTHEAST
FLANK OF THE BRIDGER RANGE, MONTANA
by
VERNE KEITH SCHRUNK
A t h e s i s s u bm itt e d in p a r t i a l f u l f i l l m e n t
o f t h e re qui re m e nts f o r t h e degree
of
MASTER OF SCIENCE
in
Earth S ci en ces
Apprbved:
C h a i r p e r s o n , Graduat’d Committee
Graduate Dfean
MONTANA STATE UNIVERSITY
Bozeman, Montana
August, 1976
STATEMENT OF PERMISSION TO COPY
In p r e s e n t i n g t h i s t h e s i s in p a r t i a l f u l f i l l m e n t o f th e r e q u i r e ­
ments f o r an advanced degree a t Montana S t a t e U n i v e r s i t y , I agree
t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r i n s p e c t i o n .
I
f u r t h e r a gr e e t h a t permi ssi on f o r e x t e n s i v e copying o f t h i s t h e s i s
f o r s c h o l a r l y purposes may be g r a n t e d by my major p r o f e s s o r , o r , in
h i s a bs e nc e, by t h e D i r e c t o r o f L i b r a r i e s . .
I t i s understood t h a t any;
copying o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l ga in s h a l l n o t be
allowed w i t h o u t my w r i t t e n pe rm is si o n .
Date
J "7^
iii
ACKNOWLEDGMENTS
The w r i t e r wishes t o e xpr e ss h is thanks to P r o f e s s o r John Montagne,
o f t h e Department o f Earth S c i e n c e s , f o r h is a i d and encouragement in
both t h e w r i t i n g and f i e l d i n v e s t i g a t i o n o f t h i s pap er.
ser ved as f a c u l t y a d v i s o r t o t h e w r i t e r .
Dr. Montagne
.
G r a te fu l acknowledgment i s extended t o Dr. Robert Chadwick. . His
c r i t i c a l reading, and comments s i g n i f i c a n t l y improved t h e pap er.
Mr.
Milton Edie , Chairman o f th e Earth S ci en ces Department, de se rv e s s p e c i a l
r e c o g n i t i o n f o r h i s l a s t minute s u b s t i t u t i o n on t h e committee.
The w r i t e r a l s o wishes t o thank h i s f e l l o w g ra d u a te s t u d e n t s , Craig
T i l l e y and John Tonnsen in p a r t i c u l a r , f o r t h e i r many h e l p f u l d i s c u s ­
s i o n s and c r i t i c i s m s .
The many land owners and t h e G a l l a t i n National F o r e s t a r e thanked
f o r allo w in g u n l i m i t e d ac c e ss to t h e i r la n d s .
In a d d i t i o n , deep a p p r e c i a t i o n and g r a t i t u d e i s extended t o my
w i f e , Sand ra, f o r her con tin ue d p a t i e n c e and encouragement throughout
t h e p e r io d o f stu dy and f o r typ in g of t h e p r e l i m i n a r y d r a f t s .
TABLE OF CONTENTS
Page
LIST OF TABLES....................................................................................
v
LIST OF FIGURES..........................
.
vii
ABSTRACT. . ............................ . ............................................ ..................................
ix
INTRODUCTION. . . ...................... ... . . :.............................................................
I
Location o f t h e A re a .......................... ........................................................ . .
I
Ph ys io gra phi c S e t t i n g .............................................................................. .... .
3
V e get a tio n and V e g e ta ti o n a l D i s t r i b u t i o n ............................................
PREVIOUS WORK ...................................
5
. . . . . . . . . . . . . . . . .
6
STRATIGRAPHY..........................................................................
G e ne ra liz e d S t r a t i g r a p h y .
.................................................................
S t r a t i g r a p h i c Summary . . . . . . . . .
...........................................
.9
.
STRUCTURAL GEOLOGY. ....................................................................................................
12
15
I n t r o d u c t i o n ......................................................................................................
15
S t r u c t u r a l U n i t s ..............................................................................
SURFICIAL GEOLOGY . . . .
.....................................................................................
18
23
I n t r o d u c t i o n .......................................................................
D e f i n i t i o n o f S u r f i c i a l Units .........................................................................
24
Colluvium ....................................................................
24
Residuum. . . . . .
25
........................................................
Talus-Rock F a l l ...........................................
26
Rock G l a c i e r . .....................................................................................
T ill.
. ..........................................................................................................................
28
29
' Page
Alluvium-Outwash.
..........................
. . . . . . .
Mass Wasting Depo sit s . ...................... ...................................... ....
REGIONAL GLACIAL CHRONOLOGY...................... .... . . .
. . .
. . .
30
30
.
32
Pr e- Bu ll Lake ................................................................................... ....
32
Bull Lake ..................................................................................................... ....
36
P i n e d a l e . ............................................................................... ....
38
Neoglaciation .
39
...................... .... .
SURFICIAL GEOLOGY OF THE MAP AREA . . . . . . .
......................
..........................
' 40-
Summary . . . . . . . . . . . . . . . . . . . . . . . . . .
.'40.'
F a ir y Creek Basin . ...................... .... . . . ' .......................... .... . . . .
Introduction.
..........................................................................; . .
Lower.Part o f F a i r y Creek Basin .......................... ....
Middle P a r t o f F a i r y Creek Basin. ................................................
Upper P a r t o f F a i r y Creek Basin . . . . . . . .
..................
42
42
44
48
53
Cache Creek Basin ............................................ ...........................................
65
Upper Forks o f B r a c k e t t C r e e k .......................... ..................................
Summary . . ■................................................................................................
North Fork o f B r a c k e t t Creek B a s i n ................................................
Middle Fork o f B r a c k e t t Creek B a s i n ................................... .... .
South Fork o f B r a c k e t t Creek B a s i n ................................................
68
68.
68
74
78
CORRELATION OF GLACIAL STADES OF THE STUDY AREA...................... .
85
OTHER GLACIATED AREAS OF THE BRIDGER RANGE.................. ....
QUATERNARY HISTORY., . ,
...................... . . . .
. . . . . . .
"..88
.
98
LAND USE CONSIDERATIONS . . . . . . . . . . . . . . . . . . .
102
REFERENCES CITED. ........................................................................ .. . . , .
107
APPENDIX:
115
Detailed S tratigraphy.
. . . . . . . . . . . . . .
,'i
vi
LIST OF TABLES
TABLE
Page
1.
Comparison o f f e a t u r e s of l a n d s l i d e s and g l a c i a l d e p o s i t s .
33
2.
Comparison o f c l a s s i c a l sequence and modified sequence
o f Wind River Mountains, Wyoming .........................
34
3.
Quater nary s t r a t i g r a p h y ....................................................
35
4.
C o r r e l a t i o n o f g l a c i a l advances o f t h e study a r e a . . . . .
87
v ii
LIST OF FIGURES
FIGURE
Page
.1.
Index map. . . . . . . . . . . . . . . . . . . . . . . . .
2
2.
View northward o f range c r e s t .............................
4'
3.
S t r u c t u r a l r e l a t i o n s o f B ri dge r Rangeregion . . . . . . .
16
T e c to n ic map o f s tu d y a r e a ...................... . ............................... .
19
5.
Panorama of. c i r q u e s o f stu dy a r e a .....................
43
6.
I n t e r m i t t e n t l a k e . .....................
47
7.
L on gi tu din al p r o f i l e o f F a i r y Creek V a lley . . . . . .
8.
F a i r y Creek t e rm in a l complex.. . . . . . . . . . . . . . . .
51
9.
T r a n s v e rs e p r o f i l e o f upper p o r t i o n o f . F a i r y Creek . . . .
52
10.
T r a n s v e rs e p r o f i l e o f middle p o r t i o n o f . F a i r y Creek. . . .
52
11.
G l a c i a l l y smoothed and rounded outc rop o f Swif t
Formation. ...................................................................
55
12.
G l a c ia l s t r i a t i o n s . . . . . . . . .
56
13.
Exposure o f t i l l ..............................
57
14.
Fairy. Lake, c i r q u e . .....................
59
15.
Older c i r q u e o f F a i r y L a k e ......................... ........................................
63
16.
Northernmost o f F a i r y Creek Basin c i r q u e s .
64
17.
Lon git ud in al p r o f i l e of Ross Peak Va lley .....................................
67
18.
Ross Peak c i r q u e ..........................................................................
69
19.
Su r fa ce e x p r e s s i o n of North Fork t i l l o f Advance I . . . .
20.
Toe o f a c t i v e l a n d s l i d e . ........................................................
. 4.
. .
............................... . . . .
. . . . . . . .
49"
' .71
72
v iii
FIGURE
Page
21.
P r o t o - c i r q u e .............................................................................
75
22.
G l a c i a l v a l l e y o f South F o r k ..........................................
79
23.
Lon git ud in al p r o f i l e o f South F o r k ................................... : .
24.
T r a n s v e rs e p r o f i l e o f Ross Peak V a l l e y .....................
25.
T r a n s v e r s e p r o f i l e of South Fork ................................... ; . .
81
26.
J u x t a p o s i t i o n o f South Fork and p r o t o - c i r q u e . . . . . .
84
27.
Cirque of North Branch o f Maynard Creek. .
28.
P r o t o - c i r q u e sou th o f stud y a r e a ..................................
91
29.
G l a c i a t e d v a l l e y s o f west s i d e o f B rid ge r Range...
92
30.
North Cottonwood Creek Canyon..........................................
93
31.
Morainal choked
32.
G l a c i a t e d upper v a l l e y o f Tom Reese Creek. . . . . . . .
96
33.
Headwaters o f Corby Canyon . . . . . . . .
97
80
81
. ...................
f l o o r o f North Cottonwood Creek . . . .
......................
.90
94
ix
ABSTRACT
The s u r f i c i a l geology o f a p a r t o f t h e n o r t h e a s t f l a n k o f th e
B ri dg e r Range, Montana in c l u d e s such d i v e r s e d e p o s i t s as t i l l , in
morainal form, co ll u v iu m , t a l u s , l a n d s l i d e d e b r i s , and residuum.
Geomorphic ev id en c e in t h e F a ir y Creek ba si n s u g g e s ts fo u r
d i s t i n c t g l a c i a l advances and r e t r e a t s . The o l d e s t advance i s
c o r r e l a t e d with t h e Bull Lake Stage o f t h e c l a s s i c a l Rocky Mountain
chronology as based on t h e fo ll ow in g c r i t e r i a a t t h i s l o c a l i t y :
1. T i l l o f t h i s age i s in a lower s t r a t i g r a p h i c p o s i t i o n than
t h e o t h e r t i l l s here.
2. The moraine r e p r e s e n t i n g t h i s advance i s f u r t h e s t downstream
from t h e c i r q u e .
3. The moraine has t h e most subdued s u r f i c i a l form.
4. There a r e fewer s u r f a c e bo uld e rs on t h e moraine o f t h i s
advance than on t h e other, moraines.
The remaining t h r e e advances a r e c o r r e l a t e d with s t a d e s I , I I , and
I I I o f t h e Pi n e d a le Stage o f th e c l a s s i c a l Rocky Mountain sequence.
As compared t o t h e Bull Lake morainal d e p o s i t s , P in e d a le d e p o s i t s
( I ) l i e s t r a t i g r a p h i c a l l y h i g h e r , (2) a r e f r e s h e r in ap pe a ra nc e ,
(3) have more bould ers exposed a t t h e s u r f a c e , (4) a r e o f l e s s e r
a r e a l e x t e n t , and (5) l i e n e a r e r t o s ourc e c i r q u e s . Cache Creek and
North Fork o f B r a c k e t t Creek c o n ta i n a l l t h e s e d e p o s i t s e xce pt f o r
t h e P in e d a le I I I s t a d e . The South Fork o f B r a c k e tt Creek, in c o n t r a s t ,
d i s p l a y s th e thre e, s t a d e s o f t h e P in e d a le Stage but no t t h e Bull Lake
Stag e.
Bedrock in t h e st udy a re a c o n s i s t s o f sedimentary s t r a t a ranging
in age from Late Devonian t o Late C re ta c e o u s , though T r i a s s i c u n i t s
a r e a b s e n t . Bedrock, where not covered by t i l l , occurs as i s o l a t e d
pa tc he s and k n o ll s and in l i n e a r r i d g e s and c l i f f s .
Land use c o n s i d e r a t i o n s acknowledge a t h r e e - f o l d d i v i s i o n o f t h e
stu dy a r e a . The we ste rn d i v i s i o n i s most s u i t a b l e f o r . i t s s c e n ic
va lu e and should not be developed. The c e n t r a l and e a s t e r n d i v i s i o n s
could be used f o r c o n s t r u c t i o n o f a l l . t y p e s , but c a u t i o n should be
used in t h e e a s t e r n d i v i s i o n because o f s t e e p s lo p e s and c o n s i d e r a b l e
q u a n t i t i e s o f b e n t o n i t e p r e s e n t in t h e bedrock and s u r f i c i a l d e p o s i t s .
INTRODUCTION
Location o f th e Area
The a r e a o f t h i s f i e l d s tu dy i s s i t u a t e d on th e n o r t h e a s t f l a n k of
t h e B rid ge r Range in n o r t h e a s t e r n G a l l a t i n County, Montana.
The
B ri dg e r Range i s l o c a t e d n o r t h and somewhat e a s t o f Bozeman, Montana.
I t i s a l i n e a r , g e n t l y curving r a n g e , approxim at el y 30 m ile s long,
forming th e e a s t e r n f l a n k o f t h e G a l l a t i n V a lle y.
The range i s the
s o u t h e a s t e r n m o s t u n i t o f t h e Northern Rocky Mountains pro vin ce of
Fenneman (1931).
S p e c i f i c a l l y , t h e s tu d y a r e a l i e s in Townships I and 2 North,
Ranges .6 and 7 E a s t , o r approximate l a t i t u d e 45o50' t o l a t i t u d e 4505 6 ' ,
approximate l o n g i t u d e IlO0SZ1 to l o n g i t u d e IlO0S?' (see f i g u r e I ) .
The a r e a o f s tu d y i s about 25 square m i l e s .
i s Montana Highway 293 o r t h e Sedan Road.
The boundary on th e e a s t
The s e c t i o n l i n e marking t h e
c e n t e r o f Township I North from t h e Sedan Road west to t h e Range c r e s t
forms th e so u th er n boundary.
The w e ste rn boundary i s t h e knif e-ed ge
c r e s t o f t h e B ri d g e r Range.
The n o r t h e r n boundary i s t h e s e c t i o n l i n e
marking t h e c e n t e r o f Township 2 North, from the Sedan Road westward to
t h e c r e s t o f t h e Range.
Primary ac c e ss t o th e e n t i r e e a s t e r n f l a n k o f th e B rid ge r Range
and t h e stu dy a r e a i s via Montana Secondary 293.
D i r e c t ac c e ss t o the.
a r e a o f stu dy i s by a few F o r e s t S e r v i c e and old logging roads and
C
'A l ___/3,
Drawn
Dept,
by E S. S m y r l
of
Earth
for
Sci ences,
Ihe
X
MSC
PHYSIOGRAPHIC
DIAGRAM
3
numerous hik in g and game t r a i l s .
Moderately dense t o dense t r e e and.
shrub growth s o me w hat .li mi ts o b s e r v a t i o n s . .
Ph ys io gr a ph ic S e t t i n g
J
The h i g h e s t peak in th e B rid ge r Range i s Sacagawea Peak a t 9665
f e e t , and i t i s w i t h i n t h e study a r e a .
On and forming t h e western
boundary i s t h e c r e s t o f t h e Range a v er ag in g from 8500 t o 9000 f e e t in
elevation.
T h e . c r e s t i s very sh arp,, almo st a ' knif e -e d g e (see f i g u r e 2 ) .
The C r e s t l i n e i s broken in only two p l a c e s ; in th e n o r th by Flathea d
P a s s , and in t h e approximate c e n t e r of t h e Range a t Ross Pass.
For t h e
most p a r t , t h i s r i d g e r i s e s a b r u p t l y from a somewhat d i s s e c t e d upland
s lo p in g t o t h e e a s t .
E l e v a t i o n s d e c r e a s e in t h e f o o t h i l l s e a s t of the
Range t o ab ou t t h e v i c i n i t y o f Montana 293.
F u r t h e r eastward from
highway 293, t h e e l e v a t i o n s r i s e s h a r p l y t o form B a t t l e Ridge and Grassy
Mountain.
The lowest p o i n t in t h e map a re a i s 5718 f e e t , and i t i s in
■
■
..
t h e extreme n o r t h e a s t e r n c o r n e r o f t h e a r e a a t Cache Creek along highway
293.
.
.
Local r e l i e f i s q u i t e g r e a t in some p l a c e s and minimal in. o t h e r s
depending upon bedrock type and t h e e r o s i o n a l p r o c e s s e s producing th e
landforms.
Seven major streams d r a i n th e a r e a .
Three of t h e s e s tr e a m s , th e
North, Middle and South Forks o f B r a c k e t t Creek, form t h e western
headwaters o f B r a c k e t t Creek.
A small p i e c e o f th e North Branch of
Figure 2. View northward from Sacagawea Peak o f th e c r e s t of the
B rid ge r Range. (Note th e l a r g e a r e a s o f s lo p e s composed of rock
ru b b le in l e f t h a n d p a r t of photo ).
5
Maynard Creek in t h e extreme s outh w est ern p a r t o f t h e st ud y a r e a flows
i n t o B ri d g e r Creek.
The o t h e r s , Cache Creek, F a ir y Creek, F r a z i e r
Creek, and t h e South Fork o f Carrol Creek, d r a i n n o r t h e a s t i n t o Fla thead
Creek which in t u r n flows eastward t o t h e S h ie ld s River and t h e Yellow­
stone.
B ri d g e r Creek.flows south through B rid ge r Canyon and t u r n s west
to t h e Eas t Fork o f th e G a l l a t i n R iv er.
These s t r e a m s , t o g e t h e r with
other processes r e f e r r e d to l a t e r , are responsible f o r the present
topography.
The only la k e s t h a t e x i s t in t h e e n t i r e B ri dge r Range, F a i r y , E l f ,
Ainger,. and F r a z i e r , a r e a l s o found w i t h i n t h e map a r e a .
E l f Lakes a r e r e a d i l y a c c e s s i b l e by au tom obile .
Only Fa ir y and.
Ainger and F r a z i e r ,
no rt h o f t h e p r e v i o u s l y mentioned l a k e s , a r e reached only a f t e r a hard
climb over f a i r l y s t e e p t e r r a i n . .
V eg eta tio n and V e g e ta ti o n a l D i s t r i b u t i o n
The v e g e t a t i o n a l p a t t e r n of t h e B rid ge r Range i s predominantly
moderate t o de n se ly f o r e s t e d a r e a s i n t e r s p e r s e d with r e l a t i v e l y open,
non-forested a r e a s .
I d e n t i f i c a t i o n o f b o t a n i c a l s p e c i e s was not a p a r t o f t h e f i e l d
work.
The f o ll o w i n g d i s c u s s i o n o f v e g e t a t i o n and i t s d i s t r i b u t i o n i s
taken from Eversman (1968).
The t r e e ta x a t h a t c h a r a c t e r i z e t h e h ig h e r a l t i t u d e s (above 7000.
f e e t ) o f t h e e a s t s i d e o f th e B rid ge r Range a r e Ticea engelmanii
6
( Engelman s p r u c e ) , Abies lasiooavya (S u b - a lp in e f i r ) , Finus f l e x i l i s
(l i m be r p i n e ) , and Populus,tvemuloides ■(Quaking a s p e n ) .
Lower a l t i t u d e t r e e s in c l u d e Pseudotsuga m en siesii (Douglas " f i r " ) ,
Pinus f l e x i l i s (Limber p i n e ) , and Populus tvemuloid.es (Quaking asp en ).
Serai t o a l l t h e s e ta x a i s Pinus oontovta (Lodgepole p i n e ) .
At no p o i n t does t h e B ri dg e r Range exceed " p h y s i o l o g i c a l ti m be rline."
However, to some e x t e n t th e v e g e t a t i o n a l p a t t e r n s a r e r e l a t e d
;
to a l t i t u d i n a l v a r i a t i o n s .
■
,
■,
.
In a d d i t i o n , t h e s e p a t t e r n s . a r e r e l a t e d t o
a complex o f m a c r o c l i m a t i c , m i c r o c l i m a t i c , h i s t o r i c a l , and to pogr a phic .
factors.
The open, n o n - f c r e s t e d a r e a s a r e covered by a combination of
Dominating t h e open a r e a s a r e Artemisia
s h r u b s , f o r b s , and g r a s s e s .
ludevioiana (Cudweed Sagewort) and
Avenaria (Sandwort).
Grasses
inc lud ed are Poa p ra ten sis (Kentucky Bluegrass), Poa spp. ( B l u e g r a s s ) ,
A grostia soabra [Ticklegrass), D aelytis glomerata ( O r c h a r d g r a s s ) , Phleum
pratense (T im oth y),
Bromus -ineramis (Smooth Brome) ,
( V i r g i n i a W i l d - r y e ) , A o h illea m illefo liu m (Yarrow),
ssissimum ( S t i c k y g e r a n iu m ) , and
.
Eltmus virgin iou s
Geranium v is io -
Arabis spp, ( R o c k c f e s s ) .
PREVIOUS WORK
McMannis (1955) has summarized t h e e a r l y g e o lo g ic work as well as
some o f t h e l a t e r work f o r t h e e n t i r e B ri d g e r Range.
The e a r l i e s t
g e o lo g ic i n v e s t i g a t i o n was done by Weed (1893), and Idd ing s and Weed
7
(1894) in p r e p a r a t i o n f o r th e L iv in gst on and Three Forks F o l i o s .
Roberts (1963-1965, 1972) mapped and d e s c r i b e d th e a r e a s d i r e c t l y south,
and e a s t o f t h e B ri dg e r Range.
e a s t e r n p a r t s o f th e Range.
Some o f h i s work a l s o covered th e s o u t h ­
Skipp and o t h e r s (1965, 1968, 1969, 1971)
mapped t h e Cretaceous a r e a s d i r e c t l y e a s t and n o r t h e a s t o f t h e Range.
A s i g n i f i c a n t s h a r e o f t h a t mapping inc lu de d t h e e a s t e r n p a r t s o f th e
p r e s e n t st udy a r e a .
Alden (1953) mentioned g l a c i a l d e p o s i t s on the
west s i d e of. t h e B r id g e r Range of p o s s i b l e pre-Wisconsin and Wisconsin
ages and reviewed t h e l a t e T e r t i a r y and Quaternary h i s t o r y of t h e Crazy
Mountains b a s i n , t h e G a l l a t i n and Yellowstone V a ll e y s .
S t r a n g e l y , he
does no t mention t h e e x i s t e n c e or t h e p o s s i b i l i t y o f g l a c i a l d e p o s i t s
on t h e e a s t e r n s i d e o f t h e Range.
McMannis (1955) acknowledges t h a t
g l a c i a l d e p o s i t s e x i s t in t h e B ri d g e rs but mapped.only t h o s e on t h e
we ste rn s i d e .
Somewhat a t odds with t h i s s t a t e m e n t i s h i s e x p la n a ti o n
o f s u r f i c i a l d e p o s i t s on t h e e a s t s i d e .
He s t a t e s t h a t both term ina l
and l a t e r a l moraines e x i s t but s u b s e q u e n tl y de nie s t h a t th e y a r e of
g la c ia l. origin.
Rather he im p li e s t h a t they a r e simply avalanche
d e b r i s (McMannis, 1955: p. 1414).
Noting- t h e d i s c r e p a n c y , t h i s t h e s i s .
was undertaken in an a t t e m p t t o a c c u r a t e l y d e f i n e and map both p o t e n t i a l
g l a c i a l d e p o s i t s and s u r f i c i a l d e p o s i t s .
STRATIGRAPHY
Since t h e bedrock geology o f t h e B ri d g e r Range was so e x c e l l e n t l y
8
mapped and d e s c r i b e d by McMannis (1955), i t would be redundan t to t r y
and r e p e a t t h i s work.
T h e r e f o r e , i t i s acc e pte d as w r i t t e n and used as
t h e p r i n c i p l e so urc e in t h e P r e - T e r t i a r y S t r a t i g r a p h i c summary to
f o ll o w .
Cretaceous rocks along t h e e a s t e r n f l a n k o f t h e Range were
mapped and d e s c r i b e d by Skipp and McGrew (T972) and f u r t h e r t o the
south and e a s t by Roberts (1972).
T e r t i a r y rocks have been t e n t a t i v e l y
i d e n t i f i e d and mapped along t h e w e s t e r n . f l a n k s of t h e Range by Hackett
and o t h e r s (1960).
These s t r a t a a r e c o r r e l a t e d with t h e Bozeman Group
o f Robinson (1959, .1961 , 1963) o f t h e c e n t r a l and w e st e rn G a l l a t i n
V a lle y.
When mapping t h e s u r f i c i a l d e p o s i t s , bedrock was noted where
i t was observed in t h e f i e l d .
The s t r a t i g r a p h y o f t h e B ri d g e r Range w i l l be d i s c u s s e d in the
p r e s e n t t h e s i s t o show i t s r e l a t i o n t o t h e for mation and e v o l u t i o n of
th e e x i s t i n g la n d s c a p e , and t o giv e t h e r e a d e r a b r i e f s k e tc h of the
bedrock geology.
The t a b u l a r s t r a t i g r a p h i c arrangement and t h e summary
to fo ll ow a r e d i r e c t e d toward t h a t g o a l .
Those rock u n i t s which
d i r e c t l y u n d e r l i e t h e s u r f i c i a l d e p o s i t s in t h e map a r e a a r e marked with
an a s t e r i s k .
Those t h a t dp not a r e s t i l l mentioned f o r completeness.
For a more d e t a i l e d s t r a t i g r a p h i c d e s c r i p t i o n th e r e a d e r i s r e f e r r e d . t o
t h e Appendix.
In a d d i t i o n t o t h e r e l a t i o n o f s t r a t i g r a p h y t o t h e for mation and
e v o l u t i o n o f t h e la n d s c a p e , bedrock hel ps t o form t h e framework and, to
an e x t e n t , t h e to p o g ra p h ic e x p r e s s io n o f t h e r e s u l t i n g s u r f i c i a l de p o s it s .
9
G e ner a liz e d S t r a t i g r a p h y .
Age
Qu aternary
Unit. Unnamed.
Thickness
0-300(?) f e e t
G e n er a li z e d Li thology
T i l l , a l l u v i u m , colluvium,
residuum, mass w a s ti n g ,
t a l u s , outwash
----------- :-------- :— U n c o n f o r m i t y --------------------- =---------------------- —
Tertiary
Bozeman
Group
0-Unknown '
Volcanic a s h , t u f f a c e o u s
m a r l , s i I t s t o n e , sandstone
and c l a y s t o n e , limes tone
Unconformity
*L iv in gs to n 9428 f e e t
Group
Volcanic s a n d s to n e , mudstone
and c onglo me rat e, welded
tu ff, altered v itric tu ff,
b e n t o n i t e , l i g n i t i c coal
*EagTe ,
Formation
100-600 f e e t
S a l t and pepper sandstone
1200-2400 f e e t
Basal s a n d s t o n e , sandstone
and-siltstone
386-447 f e e t
Basal cong lom erat ic
s a n d s to n e , c l a y s t o n e and
s h a le
Cretaceous.
. ^Colorado
Group
*Kootenai
Formation
Disconformity
*Morrison
Formation
110-444 f e e t
Shale and mudstone,
s a n d s to n e , upper p a r t
carbonaceous .
Jurassic
* E l l i s Group 70-359 f e e t
(S w if t Formation
Rierdon Formation .
Sawtooth Formation)
Sandstone
Limestone, s h a l e and s i l t s t o n e ; Shale and mudstone,
sandstone
10
------------------------------------------------ Disco nfo rmity —------------- ------ :— ■
— ------------- Permian
Phosphoria
0-26 f e e t .
Chert n o d u le s , c h e r t b r e c c i a s
and conglomerates .
------------------- ———------------:-------- Disconformity —---------------------- -—
Pennsylvanian *Quadrant
Formation.
1.53-250 f e e t
.
Mis s i SSi p p i an-*Amsden'
Pennsylvanian Formation
11-189 f e e t
Dolomite with q u a r t z
s a n d s t o n e s . i n lower p a r t ,
. dolomite bed s, pure q u a rt z
sand st one o r q u a r t z i t e
S i l t s t o n e and dolo m ite ,
. s h a l e with in te rb e d d e d
li me st one
------------- :— ---------------------- :------Disconformity ------------------------------:------------:----*Big Snowy
Group
0-434 f e e t
Basal dolomite or limes tone
b r e c c i a , s i l t s t o n e and
d o l o m i t e s , s an ds to ne or
c a l c a r e n i t e , b la c k sh aly
lim e s to n e and s h a l e
—— :----------- :---------- :— ^---------Disconform ity--------------- :--------- :-----------------M i s s i s s i p p i a n *Madison Group 1188-1760 f e e t
( Lodgepole Formation
Basal block s i l t y s h a l e ,
thi nbedded lim e s to ne
Mission Canyon Formation) Limestone, s o l u t i o n b r e c c i a ,
s o l u t i o n c h a n n e l s , caves
a t top
------------- :------------------------:----------Disco nfo rmity =----------------------------------------------*Sappington
Formation
46-99 f e e t
Basal b la ck s h a l e , sa n d s to n e .
s i l t y and sandy limestone
*Three Forks
Formation .
155 f e e t
Basal e v a p o r i t e s o l u t i o n
b re c cia , brecciated limestone,
s i l t s t o n e and s h a l e
11
Devonian
*Jefferson
Formation
497-620 f e e t
Dolomite, d o l o m i t i c li m e s t o n e .
l i m e s t o n e , and in te rbe dded
dolomitic s i l t s t o n e
*Maywood
Formation
39-92 f e e t
Mudstone and s i l t s t o n e .
dolomite beds
Disconformity
Cambrian
Snowy Range
Formation
167-280 f e e t
F i s s i l e s h a l e w ith i n t e r ­
bedded s i l t s t o n e and sandstone,
columnar l i m e s t o n e , lim es ton e
and lim e s to n e pebble
conglomerate
Pilgrim
Formation
360-433 f e e t
Basal o o l i t i c l i m e s t o n e , f l a t pebble li m e s to n e conglomerate
and s h a l e , o o l i t i c limestone
Park
Formation
188-192 f e e t
Arkose and a r k o s i c li me st one
(lower p a r t ) and f i s s i l e shale
and lim e s to n e (upper p a r t )
Meagher
Formation
370 f e e t
Dense li m e s to n e with
i n t e rb e d d e d s h a l e and s i l t y
shale
Wolsey
Formation
152-210 f e e t
Micaceous s h a l e and s a n d s to n e ,
l o c a l l y c o n t a i n s a r k o s i c and
c onglo me rat ic a r k o s i c lime­
st o n e
Fla th e a d
Formation
119-142 f e e t
Sandstone, l o c a l l y q u a r t z i t i c
and a r k o s i c
Unconformity
Belt
7-10,000 f e e t Coarse con glom erate, a r k o s i c
Supergroup
s a n d s to n e , a r g i l l i t e ,
(LaHood Formation)
s i l i c e o u s lim e s to n e
12
.
------------------------------------------ ------- -- F a u l t ----------:-------------- :----- —
Precambrian:
--------------
:
Metamorphic
Basement
Complex
Unknown
Quartz-feldspar gneiss.
. g a r n e t- a m ph ib ote s c h i s t ,
d o l o m i t i c m a rb le , pegmatite
S t r a t i g r a p h i c Summary
The Precambrian, Cambrian, and T e r t i a r y s t r a t a crop out only on t h e
we ste rn f l a n k s o f t h e Brudger Range and do not c o n t r i b u t e t o t h e s u r f i c i a l deposits of the e ast side.
a t th is point.
T h e r e f o r e , they w i l l not be d is c u s s e d
(For a d i s c u s s i o n of t h e s e s t r a t a see Appe ndix .)
Devonian rocks a r e fou nd-only in a very small s l i c e in t h e extreme
n o rt h w e s te r n c o r n e r o f t h e s tu dy a r e a in t h e headwall o f t h e c ir q u e
upstream from F a ir y Lake ( s e e P l a t e I ) .
These a r e t h e Maywood, J e f ­
f e r s o n , Three F o r k s , and Sappington Formations.
The J e f f e r s o n i s the.
only r e s i s t a n t , ledge o r r i d g e forming u n i t ; t h e r e s t a r e n o n - r e s i s t a n t .
The l i t h o l o g i e s t h e y r e p r e s e n t , mudstone, s i l t s t o n e , and s h a l e , account ■
for t h e i r n o n -re s is ta n t nature.
All a r e e a s i l y eroded and a re
e x p re ss e d in t h e lan dsc ap e as swales where no t t o p o g r a p h i c a l l y high
because o f a s s o c i a t i o n with more r e s i s t a n t f o r m a ti o n s .
/
The Madison Group, o f M i s s i s s i p p i a n Age comprises t h e Lodgepole and
Mission. Canyon Formation's.
o f t h e e n t i r e Range.
These u n i t s form t h e k n if e -e d g e d backbone
T y p i c a l l y , t h e y form t h e c a s t e l l a t e d r i d g e s ,
l e d g e s , and c l i f f s , in much of western Montana. . Madison Group rocks a r e
13
found in and on t h e moraines as huge blocks as well as some s m a l le r
fractions.
In f a c t , th e y comprise t h e m a j o r i t y o f bou ld e rs on t i l l
surfaces.
Also , t h e m a j o r i t y o f t a l u s in t h e stud y a r e a i s composed
o f Madison Limestone.
Big Snowy s t r a t a , a l s o of M i s s i s s i p p i a n Age, a r e n o t r e a d i l y
a p p a r e n t in t h e B ri d g e r Range (McMannis, 1955).
This i s not s u r p r i s i n g
s i n c e th e y c o n s i s t predomin an tly o f s h a l e and s i l t s t o n e which a re
e a s i l y removed by e r o s i o n and vary g r e a t l y in t h i c k n e s s .
The Amsden Formation, o f M i s s i s s i p p i a n - P e n n s y l v a n i a n Age, i s the
most e a s i l y rec og ni ze d for matio n in t h e B r i d g e r s .
I t i s d i s p l a y e d as
a conspicuous r e d d i s h - c o l o r e d swale between t h e more r e s i s t a n t Madison
Group and Quadrant Formation.
The Quadrant Formation forms t h e ba sal p a r t s of t h e f l a t i r o n s , along
t h e f l a n k s o f t h e Range.
I t i s a very r e s i s t a n t fo rm ati on composed
mostly o f q u a r t z s a n d s to n e .
Large bou ld e rs of Quadrant can o f t e n be
found s c a t t e r e d on t h e s u r f a c e o f t h e mora ine s.
The bo uld e rs a r e not
as numerous as t h o s e of t h e Madison Group, however.
The Permian Phosphoria Formation i s r e p r e s e n t e d by an extremely
t h i n , very l o c a l accumulation of c h e r t , c h e r t b r e c c i a , and c h e r t
c o n g l o m e r a t e . . I t i s d i f f i c u l t to l o c a t e in outc rop and i s not found in
any o f t h e s u r f i c i a l . d e p o s i t s .
J u r r a s s i c s t r a t a a r e d e p i c t e d by t h e E l l i s Group and t h e Morrison
Formation.
The E l l i s Group in c l u d e s t h r e e f o r m a t i o n s , t h e Sawtooth,
14
R ie rd o n , and t h e S w if t.
The Rierdon and S w if t a r e r e s i s t a n t and stand
as r i d g e s where not covered by t i l l o r mass wa sting d e p o s i t s oh removed
by i c e .
Pebble and c o b b l e - s i z e d p i e c e s , r a r e l y b o u l d e r s , o f t h e s e
fo rm at io ns a r e found in t h e t i l l .
Ex te ns iv e a r e a s o f t h e e a s t f l a n k o f t h e B rid ge r Range a r e under­
l a i n by t h e Morrison Formation.
I t i s ve ry n o n - r e s i s t a n t due to th e
l a r g e q u a n t i t i e s o f s h a l e and mudstone p r e s e n t .
f o r e s t e d a r e a s a r e u n d e r l a i n by t h e f o r m a ti o n .
a r e t h e two most common s u r f i c i a l d e p o s i t s .
Many o f t h e l a r g e nonResiduum and colluvium
Where t h e Morrison i s
e x p o s e d .i n a s t e e p s l o p e along t h e v a l l e y s i d e s , mass wa sti ng d e p o s i t s
a r e common.
The basal s an ds to ne o f t h e Qretaceous Kootenai Formation forms
small r i d g e s thro ugho ut t h e st udy a r e a where not co ncealed by t i l l .
It
a l s o c o n t r i b u t e s both l a r g e blocks and s m a l l e r s i z e blocks o f sandstone
to th e t i l l .
These blocks a r e no t as p l e n t i f u l as th o s e o f t h e Madison
Group and a r e about as p l e n t i f u l as t h o s e o f t h e Quadrant Formation.
Large amounts o f sand , s i l t , and c l a y - s i z e d p a r t i c l e s a re
c h a r a c t e r i s t i c e r o s i o n pro d u c ts o f t h e s an ds ton es and s i l t s t o n e s from
t h e Colorado Group.
The outc rop a r e a o f t h i s u n i t i s l a r g e l y the
d e p o s i t i o n a l s i t e o f t i l l from t h e g l a c i e r s and, t h e r e f o r e , t h e rocks
a r e covered.
Where th e y a r e n o t , mass w a s t i n g , c o ll uv iu m , and residuum
a r e t h e s u r f i c i a l d e p o s i t s a s s o c i a t e d w it h t h e Colorado Group.
.
15
The e n t i r e e a s t e r n p a r t o f t h e s tu dy a r e a i s u n d e r l a i n by the
Li vin g st o n Group or i t s e q u i v a l e n t s (s e e d i s c u s s i o n in Appendix).
major p o r t i o n o f t h i s , a re a was no t reached by i c e .
1
The
Consequently, the
'
-
••
s u r f i c i a l d e p o s i t s c o n s i s t o f in s i t u weathered pro duc ts o f th e bedrock.
There a r e , however, numerous exposures o f bedrock surrounded by the
w eat he rin g d e b r i s .
The L iv in gs to n s t r a t a a r e composed mainly o f volcanic
d e t r i t u s which i s r e a d i l y a t t a c k e d by w e at h e ri n g .
The weath ering prod­
u c t s a r e c l a s s i f i e d e i t h e r as residuum o r as colluvium i f downs!ope
movement has taken p l a c e .
STRUCTURAL GEOLOGY
Introduction
An u n d e rs ta n d in g and d e l i n e a t i o n o f th e s t r u c t u r e and s t r u c t u r a l
r e l a t i o n s h i p s o f t h e North B rid ge r Range i s n e c e s s a ry because t h e s e
r e l a t i o n s h i p s a i d in i n t e r p r e t i n g t h e t y p e , development, and placement
of the s u r f i c i a l d e p o sits.
The s t r u c t u r a l f e a t u r e s in t h e Bridg er
Range, to a l a r g e d e g r e e , c o n t r o l e l e v a t i o n and t r e n d s o f t h e rock,
th us i n f l u e n c i n g such t h i n g s as stream placement and development, t h e
d i r e c t i o n o f i c e flow , t h e s t e e p n e s s o f s l o p e s and t h e i r o r i e n t a t i o n ,
as well as some weakness zones in t h e rocks th emselves.
The s t r u c t u r a l e n t i t i e s of. th e B ri d g e rs and u n i t s o f both loc al
and re g i o n a l e x t e n t have been d e t a i l e d b y .McMannis (1955).
Related
a d j a c e n t a r e a s have been d i s c u s s e d by Roberts (1972), G a r r e t t (1972),
16
LEOEND
Thrust Feult, Borbe on
horn@tng well
A
Overturned Thrust
teeth in dip direction
Anticline
Overturned Anticline
Overturned Syncline
Adopted from R o b e rts(1972), and
McMonnIs B Shipp (1908)
BOZEMAN
Figure 3. S t r u c t u r a l R e l a t i o n s of B ri dge r Range region (Study area i s
o u t l i n e d by a dashed p a t t e r n ) .
17
McMannis and Chadwick (1964), Robinson (1959, 1961, 1963), Klepper e t
a l . (195 7) , Klemme (1949), Hackett e t a l . (1960), and Weber (1965).
The B ri d g e r Range o r i g i n a t e d during t h e d i a s t r o p h i c a c t i v i t y known
as t h e Laramide Orogeny.
I n i t i a l t e c t o n i c a c t i v i t y began in Late
Cretaceous ti m e .a n d die d out during Oligocene time.
Over t h i s time
span two major p u l s e s o f d ia s t r o p h i s m were r e s p o n s i b l e f o r f a u l t i n g ,
f o l d i n g , and e l e v a t i o n o f t h e Range (McMannis, 1955).
Robinson (1961
and 1963) and Roberts (1972) a l s o i n d i c a t e two major e p is o d e s o f .
tectonic a c tiv ity .
Roberts (1972) concurs with McMannis in t h e
s p e c i f i c s o f tim e.
Robinson (1961 and 1963), however, b e l i e v e s t h a t
t h e major t e n s i o n a l phases oc cu rr ed from mid t o Late Eocene through
E a rly Oligocene and l a t e r again in Late Miocene through P li o c e n e .
McMannis (1955) b e l i e v e s t h e f i r s t p u l s e , a west to e a s t, compression,
in mid-Paleocene r e s u l t e d in some e l e v a t i o n of t h e Range but more in
r e a c t i v a t i o n o f old f a u l t s and pro d u c ti o n o f f o l d s .
The second p u l s e ,
•in Late Paleocene or Ear ly Eocene, a s o u th -s o u th w e s t t o n o r t h - n o r t h e a s t
compression, r e s u l t e d in c ont inu ed f a u l t movement, general, under­
t h r u s t i n g and f u r t h e r deformation and, in some c a s e s , o v e r t u r n i n g of
existing folds.
I s d s t a t i c arching, durin g Oligocene time produced
normal f a u l t s on t h e west s i d e o f t h e Range and ddwndropped t h e G a ll a t in
V a ll ey .
Pardee (1950) b e l i e v e s t h a t t h e r e has been a t l e a s t 3000 f e e t
o f movement along t h e s e normal f a u l t s .
18
Major s t r i k e - s l i p f a u l t s d i v i d e t h e . Bridg er Range i n t o s t r u c t u r a l
segments.
The northe rnmo st f a u l t i s t h e Cross Range F a u l t which
o r i g i n a t e d and was a c t i v e during t h e Laramide Orogeny.
The second
f a u l t , a l s o o f s t r i k e - s l i p n a t u r e , i s f a r t h e r south and in. r e a l i t y a
f a u l t zone.
This f a u l t zone i s composed o f t h r e e f a u l t s , t h e Ross
Peak, t h e P a s s , and t h e Dry Fork f a u l t s .
The Pass F a u l t was a c t i v e
during Precambrian t i m e , l o c a l i z i n g d e p o s i t i o n of t h e B e l t i a n arkoses
and conglomerates (McMannis, 1955).
The Ross Peak and Dry Fork F a u l t s
o r i g i n a t e d duri ng t h e Laramide Orogeny.
In t h e so ut her n p a r t o f the
Range a r e o t h e r s i m i l a r major f a u l t s b u t , as t h e s e had l i t t l e or no
e f f e c t on t h e s u r f i c i a l d e p o s i t s o f t h i s r e p o r t , th e y w i l l not be
discussed.
The s t r u c t u r a l segments and t h e i r r e l a t e d f a u l t s and f o l d s had a
d i r e c t e f f e c t on t h e t y p e , e v o l u t i o n , and placement o f t h e s u r f i c i a l
deposits.
With t h i s ide a in mind, each segment w i l l be e xplo re d more
f u l l y as a background o r s k e l e t a l framework f o r th e s u r f i c i a l d e p o s i t s .
The segments t o be d i s c u s s e d , t h e North B ri dge r segment and th e
Sacagawea segment (McMannis, 1955), a r e t h e l o c i o f t h e s u r f i c i a l
deposits of t h i s rep o rt.
S t r u c t u r a l Units
The North B ri d g e r segment i s bounded on t h e south by t h e Cross
Range F a u l t (s e e f i g u r e 4 ) .
On t h e n o r t h , e a s t , and west s i d e s , th e
19
Ainger Loke
Foiry Lake
Brackett Creek
Legend
U
Ro”
(Upthrown Block)
D
Fault
Bottle Ridge Fault
( Downthrown Block)
Anttclina
4-
Syncline
O
I
MuT
X
Adapted from Skipp and McMannis (1968)
Figure 4.
T ec ton ic map of study a r e a .
t
f
20
l i m i t s o f t h i s segment c o i n c i d e with t h e Range l i m i t s .
s t r u c t u r a l f e a t u r e i s t h e Cross Range F a u l t ,
The major
I t . c a n be t r a c e d from
S e c ti o n I , T 2 N, R 5 E on t h e north w es t s i d e o f t h e Range t o Fairy
Creek.
I t can p o s s i b l y be t r a c e d d i r e c t l y n o r th o f t h e f o r k s of
B r a c k e t t Creek and along t h e s outh er n f a c e o f B a t t l e Ridge.
The f a u l t
i s r a r e l y exposed but i s conspicuous as i t c r o s s e s t h e Range in the
v i c i n i t y o f Ainger and Bighorn Lakes.
Movement along t h e f a u l t com­
bi n e s s t r i k e - s l i p with d i p - s l i p , t h e n o r t h e a s t block having moved
s o u t h e a s t and t h e southwest block n o r t h w e s t , u n d e r t h r u s t i n g t h e n o r t h ­
e a s t block (McMannis , 1955).
The a t t i t u d e s o f t h e beds ( s t e e p l y dippin g t o o v e r t u r n e d ) and the
a n t i c l i n e s and s y n c l i n e s in t h e e a s t and north w es t p a r t o f t h i s segment
a r e r e l a t e d t o t h i s major s t r u c t u r e .
These f e a t u r e s a r e b e li e v e d to
have been produced by u n d e r t h r u s t i n g o f t h e North B rid ge r segment by
t h e block t o t h e southwest o f t h e Cross Range F a u l t .
■
.
7
D i a s t r o p h i c a c t i v i t y i s r e l a t e d t o two e p i s o d e s .
'
The f i r s t
produced t h e l a r g e s c a l e f e a t u r e s , i . e . , a l a r g e n o r t h e a s t plunging
a n t i c l i n e forming t h e North B rid ge r segment and t h e sag s e p a r a t i n g t h e
B ri dg e r Range from t h e Big B e l t Mountains and some n o r t h e a s t t r e n d in g
f o l d s (Elkhorn A n t i c l i n e ) , t o the. n o r t h .
The second e p is o d e of
a c t i v i t y , compression from s o u th - s o u th w e s t t o n o r t h - n o r t h e a s t , r e s u l t e d
in u n d e r t h r u s t i n g o f t h e North B ri d g e r b l o c k , o v e r t u r n i n g o f beds, drag
21
along t h e Cross Range F a u l t , d e f l e c t i o n , and squeezing t o g e t h e r o f th e
f o l d s (McMannjs, 1955).
■ The Sacagawea segment i s southwest o f t h e North B ri d g e r segment,'
; .
t
■'
'
wit h t h e Cross Range F a u l t as i t s n o r t h e r n boundary. T h e .so uthern
boundary i s t h e f a u l t zone composed o f t h e Ross Peak, P a s s , and Dry
Fork f a u l t s .
The west ern and e a s t e r n bounda ries o f t h i s segment a re
t h e l i m i t s o f t h e Range.
'
'
'
■
At
t h e c r e s t and eastward from i t th e
.
' .
. .
P a le o z o i c and Mesozoic sediments are, mo derat el y t o s t e e p l y dipp ing.
In t h i s r e s p e c t , t h e a r e a i s l e s s complex
segment.
However, due t o t h e p resen ce
than th e
of t h e Pass
segment i s s t r u c t u r a l l y t h e most complex.
North Bridg er
F a u l t zone, t h i s
The Pass F a u l t was a c t i v e
du rin g Precambrian time l o c a l i z i n g d e p o s i t i o n of t h e B e l t i a n arkoses
and conglomerates as mentioned above.
Movement along Pass F a u l t has
been o f two t y p e s , d i p - s l i p and s t r i k e - s l i p .
This movement on th e
s o u t h e a s t end has been r e v e r s e d from t h a t o f Precambrian.
Precambrian
movement brought t h e southwest block up and t h e n o r t h e a s t block down.
Laramide movement developed t h e o p p o s i t e r e l a t i o n s :
e a s t block down and t h e n o r t h e a s t up.
i . e . , the south­
This f a u l t may have been a c t i v e
du rin g E a rly P e n ns ylv an ia n-L at e M i s s i s s i p p i a n and a l s o durin g J u r a s s i c
time (McMannis, 1955).
The Pass F a u l t i s a l s o p o s t u l a t e d as a nort hw es t c o n t i n u a t i o n of
t h e Nye-Bowler Zone (McMannis, 1955).
G a r r e t t (1972) s u g g e s t s t h a t
.
t h e Pass F a u l t , r a t h e r than t h e Cross Range F a u l t extend s eastward from
22
t h e B rid ge r Range underneath B a t t l e Ridge.
B a t t l e Ridge i s a monocline
subtended by a major s o l e t h u r s t ( G a r r e t t , 1972; R o b e r ts , 1972).
The
c o n t i n u a t i o n o f t h e Nye-Bowler Zone i s not through t h e Pass F a u l t but
r a t h e r , f u r t h e r s o u t h , n e a r t h e so u th er n margin of t h e B ri d g e r Range
(Bear Canyon F a u l t - C h e s t n u t Mountain A n t i c l i n e ) ( G a r r e t t , 1972; R ob er ts ,
1972).
Westward from t h e B ri dg e r Range, Robinson (1961) p o s t u l a t e s
t h a t t h e i n f e r r e d Willow C r e e k . F a u l t i s a p o s s i b l e e x t e n s i o n o f th e
Pass F a u l t .
Ha ckett and o t h e r s (196 0) , c a l l t h i s i n f e r r e d f a u l t the
C ent ral Park F a u l t and extend i t t o t h e B ri d g e rs through Spring H i l l .
This f a u l t was a l s o a c t i v e in Precambrian time l o c a l i z i n g d e p o s i t i o n o f
t h e B e l t i a n LaHood Formation.
Ross Peak F a u l t , n o r th o f t h e s u b p a r a l l e l t o Pass F a u l t , i s of
Laramide (?) o r i g i n and a l s o shows s t r i k e - s l i p movement with t h e no rth
block moved s o u t h e a s t and up r e l a t i v e t o t h e south bloc k. .
The o t h e r s t r u c t u r e s . in t h i s segment a r e a s e r i e s o f a n t i c l i n e s
and s y n c l i n e s d i r e c t l y e a s t and t r e n d i n g n o r t h - n o r t h e a s t of Ross Peak.
These s t r u c t u r e s a r e t r u n c a t e d on t h e i r so ut her n end by t h e Ross Peak
Fault.
The f o l d s a r e b e s t exposed in and ne ar t h e l a r g e S curves in
'
.
t h e Mesozoic s t r a t a bu t can be d i s c e r n e d in th e Cretaceous s t r a t a with
some d i l i g e n t s e a r c h i n g .
These f o l d s a r e no t d i r e c t l y r e l a t e d t o t h e
f a u l t s as a r e t h e f o l d s o f t h e North B ri d g e r segment.
R a t h e r , they
a r e b e l i e v e d t o be r e l a t e d t o t h e l a t e phase o f . Paleocene deformation
(second p u l s e ) .
.
23
The s t r u c t u r a l h i s t o r y o f t h e n o r t h e r n two segments o f th e Bridger
Range, t h e n , can be summarized a s :
I ) a c t i v e movement o f t h e Pass
F a u l t in Precambrian t i m e , l o c a l i z i n g d e p o s i t i o n of B e l t s t r a t a (LaHood
F o r m a t io n ) ; 2) submergence with accompanying d e p o s i t i o n ; 3) p o s s i b l e
r e a c t i v a t i o n along t h e Pass F a u l t in Late M i s s i s s i p p i a n - E a r l y Pennsyl­
vanian ti m e ; 4) i n a c t i v i t y , submergence, and d e p o s i t i o n ; 5.) renewed
t e c t o n i c a c t i v i t y in Late Cretaceous with l a r g e s c a l e f o l d i n g ; 6) th e
f i r s t v i o l e n t p u ls e o f o r o gen ic a c t i v i t y in Early Paleocehe with
a c t i v e s t r i k e - s l i p movement and some f o l d i n g , a period, o f quiescence
accompanied by e r o s i o n ; 8.) th e second p u l s e o f a c t i v i t y in Late
P a le o c e n e -E a rl y Eocene w i t h . g e n e r a l u n d e r t h r u s t i n g and f u r t h e r accen­
t u a t i n g o f f o l d s ; 9) i s o s t a t i c (?) a r c h in g in Oligocene with normal
f a u l t i n g on t h e west s i d e ; and f i n a l l y 10) c o n t i n u e d . a c t i v i t y along t h e
west s i d e o f t h e B ri d g e r Range w ith normal f a u l t i n g c o n t i n u i n g i n t o
Late P l i o c e n e - E a r l y P l e i s t o c e n e .
SURFICIAL GEOLOGY
Introduction
..
' '
The m a j o r i t y o f f i e l d time was s p e n t in mapping s u r f i c i a l d e p o s i t s .
Bedrock was noted and p l o t t e d during t h e course o f f i e l d , but a study
o f bedrock i s not t h e major o b j e c t i v e . . The p o r t i o n s of. t h e map d e s i g ­
n a te d as bedrock a r e th o s e a r e a s where t h e bedrock i s exposed, or where
t h e s u r f i c i a l u n i t s a r e so s p a r s e as t o make them unmappable.
U su a lly ,
24
a r e a s mapped as bedrock a r e u n d e r l a i n by t h e more r e s i s t a n t rock types
and g e n e r a l l y a r e t o p o g r a p h i c a l l y e x p re ss e d as r i d g e s o r k n o l l s .
The fo ll o w i n g s e c t i o n on s u r f i c i a l d e p o s i t s in c l u d e s an explanation
of t h e mapping u n i t s used and a b r i e f s k e tc h o f t h e development of
Quaternary g l a c i a l chronology and te rm in olo gy.
F i n a l l y , each dra in a g e
i s c o n s id e re d and i t s . s u r f i c i a l d e p o s i t s i n t e r p r e t e d .
D e f i n i t i o n o f S u r f i c i a l Units
The s u r f i c i a l u n i t s enumerated and d e s c r i b e d below a r e th o s e u n i t s
determined by t h e w r i t e r t o be t h e s i m p l e s t c a t e g o r i e s and th o s e most
e a s i l y d e l i n e a t e d and u s ab le in t h e f i e l d .
Many d e p o s i t s pla ced in one
c a t e g o r y could e a s i l y have been a s s ig n e d t o a n o th e r .
The d i s c r e t i o n ..
o f t h e w r i t e r and s i m p l i c i t y were t h e b a s i s o f c hoic e .
Colluvium
j.
Colluvium o r c o l l u v i a l d e p o s i t s c o n s i s t o f f i n e t o c o a r s e - r o c k and
"
s o i l d e b r i s which i s i n c o h e r e n t o r u n c o n s o l i d a t e d .
Commonly, i t i s
de ri v e d from t o p o g r a p h i c a l l y and s t r a t i g r a p h i c a l l y h ig h e r u n i t s but may
i n c lu d e i n - p l a c e r e s i d u a l m a t e r i a l .
As used in t h i s p a p e r , t h e term
colluvium r e f e r s t o m a t e r i a l p a r t l y or mostly made up o f p a r t i c l e s
p e b b l e - s i z e d and s m a l l e r , a n g u l a r as well as rounded, o r m a t e r i a l having
a framework with a m a tr ix o f s m a l l e r s i z e s .
Thus, colluvium i s composed
o f m a t e r i a l s loosened from bedrock by w eat herin g p r o c e s s e s and subse­
q u e n t l y moved t o o r toward t h e base of t h e sl o p e where the m a t e r i a l s
25
accumulate (residuum) or a r e c a r r i e d away by f l u v i a l mechanisms./
As"a
r e s u l t , colluvium merges a lm os t i m p e r c e p t i b l y with a d j a c e n t s u r f i c i a l
d e p o s i t s , and c o n t a c t s a r e o f t e n vague and u n c e r t a i n .
Consequently,
colluvium u n i t s may be c o n t r a s t e d with residuum o r m a t e r i a l formed by
w e at he rin g in s i t u (s e e below).
Included in t h i s c a t e g o r y , f o r mapping p u r p o s e s , a r e some slow t o
very slow moving mass wast ing phenomena.
Therefore, creep, soil creep,
and rock creep d e p o s i t s a r e a l s o inc lu de d in t h i s g en eral u n i t .
Talus'
and c l i f f d e b r i s a r e not mapped under t h e heading.
Residuum
Residuum, r e g o l i t h , or r e s i d u a l m a t e r i a l , as used in t h i s r e p o r t
and as a mapping u n i t , in c lu d e s t h a t d e b r i s which i s l o o s e , in c oher en t
s o i l forming m a t e r i a l .
I t i s m a t e r i a l t h a t i s e s s e n t i a l l y in p la ce o r ,
a t b e s t , has moved only a r e l a t i v e l y s h o r t d i s t a n c e (compared to stream
t r a n s p o r t d is t a n c e s ) , from i t s so urc e and has been r e d e p o s i t e d .
Col­
luvium does no t show any f e a t u r e s t o s u g g e s t d e p o s i t i o n by streams.
I t has and c o n t i n u e s t o undergo both ph y s ic a l and chemical w eathering .
Chemical w e at he ri ng dominates.
Thickness o f t h e d e p o s i t s ranges from
a pp ro xi m a te ly s i x inches on f l a t or v e ry g e n t l y s lo p i n g s u r f a c e s to s i x
o r more f e e t along t h e bases o f s t e e p s l o p e s and in some s w a l e s ;
Deposits showing t h e beginning s t a g e s o f s o i l for mation to th os e
showing moderate s o i l p r o f i l e a r e inc lud ed in t h i s c a t e g o r y .
Soil i s a
26
n a t u r a l body o f mineral and o r g a n ic m a t t e r which changes or. has changed
in resp on se t o c l i m a t e and organisms.
A s o i l e x h i b i t s t h r e e dimen­
s i o n a l sequence o f c h a r a c t e r i s t i c s (Buol, Hole, McCracken, 1973).; The
v e r t i c a l dimension and such c h a r a c t e r i s t i c s a r e t h e s o i l p r o f i l e .
a l s o e x h i b i t l a t e r a l f a c i e s changes.
Soils
T h u s , t h e m a t e r i a l mapped herei n
as residuum d i s p l a y s s t a g e s o f very p oorly developed p r o f i l e s in a r e a s
o f shallow d e p o s i t s t o a somewhat b e t t e r developed p r o f i l e in th e
t h i c k e r and w e t t e r a r e a s .
'
Np d e t a i l e d examination o r stud y was made of. t h e r e s i d u a l d e p o s i t s
or t h e i r associated s o i l s .
The e x t e n t o f examination f a c i l i t a t e d
d i s t i n c t i o n between co llu vi um, residuum, and o t h e r mapping u n i t s .
T a lu s -R o c k f a ll
T a l u s , in t h i s r e p o r t , i s d e fi n e d as a n g u l a r , l a r g e - s i z e d rock
d e b r i s a d j a c e n t t o a steep, c l i f f o r rock ou tc ro p .
No f lo w a g e .o r o t h e r
movement has taken p l a c e o t h e r than t h a t which was imparted by rocks
f a l l i n g and r o l l i n g t o a s t a b l e p o s i t i o n o r one o f very l i m i t e d
mobility.
R oc kf al I d e b r i s o r i g i n a t e s from v e r t i c a l or overhanging c l i f f s and
very s t e e p r o c k w a l ls .
Blocks o r p l a t e s o f rock a r e loosened p e r i o d i ­
c a l l y by w e at he rin g and drop away under t h e i n f l u e n c e o f g r a v i t y .
The t a l u s and r o c kfa 11 d e b r i s most l i k e l y o r i g i n a t e s by f r o s t
wedging o f t h e i n d i v i d u a l blocks along l i n e s o f s t r u c t u r a l weakness
27
formed by f a u l t s , j o i n t s , and a l s o by weaknesses and a b n o r m a l i t i e s in
crystal stru c tu re .
Weakened and loosened by f r o s t wedging, th e s e
b l o c k s , where s u f f i c i e n t l y f r e e from o b s t r u c t i o n s , tumble, r o l l , and
f r e e l y f a l l t o p i l e s and s lo p e s below c l i f f s and rock o u t c r o p s .
It
i s i n t e r e s t i n g t o note , t h a t t h e v a s t m a j o r i t y o f t h e t a l u s , s lo pes in ...
t h e map a r e a a r e composed o f l im e s to n e .f ro m th e Madison Group.
Other formations, have very small t a l u s d e p o s i t s o r none a t a l l .
T a l u s , as d e f i n e d , in c l u d e s some s m a l l , l o c a l l y . d e v e l o p e d p r o - t a l u s
ra m p a rt s .
A p r o - t a l i i s rampart i s a r i d g e of rock w a s te , u s u a l l y
composed o f f a i r l y l a r g e , a n g u l a r bloc ks t h a t have been moved to t h e i r p r e s e n t p o s i t i o n by r o l l i n g over p r e e x i s t i n g snowbanks ( F l i n t ,
1971).
These r i d g e s a r e g e n e r a l l y found in and around c i r q u e s , a t
t h e edges o f snowbanks and n i v a t i o n hollows , and sometimes a t t h e
bases o f c l i f f s . . In t h e B r i d g e r s , d e p o s i t s i n t e r p r e t e d as p r o - t a l u s
ramparts a r e co nfin ed t o t h e c i r q u e s and bases o f some o f t h e s t e e p .
c l i f f s a t t h e n o r th o f F a i r y Lake.
P r e s e n t l y , they appear more l i k e
knob and k e t t l e topo gra phy, some having t h e form of l i n e a r arid g e n t l y
curving r i d g e s but w it h o u t much f i n e m a t e r i a l .
This to po g ra p h ic
e x p r e s s i o n i s b e l i e v e d t o have r e s u l t e d from t h e m e lt in g o f snowbanks,
i c e , and f r o s t with subsequent c o l l a p s e o f t h e a s s o c i a t e d rock ru b b le . ;
Also,
con tin ue d f e e d in g of d e b r i s from t h e nearby overhanging c l i f f s
added m a t e r i a l which helped t o subdue and a l t e r t h e o r i g i n a l shapes.
28
Rock G l a c i e r
The oc cu r re n c e o f a c t i v e rock g l a c i e r s o r i n c i p i e n t rock g l a c i e r s
i s l i m i t e d t o t h e c i r q u e above F a i r y Lake.
Deposits b e l i e v e d to have
once been rock g l a c i e r s a r e found in t h e c i r q u e s to t h e n o r th and south
o f F a ir y Lake, however.
A rock g l a c i e r i s a g e n e r a l l y l o b a t e , s p a t u l a t e , or d i g i t a t e ­
shaped accumulation o f a n g u l a r , bbuld ery r u b b l e .
i n t e r s t i t i a l Iy held a t some dep th.
Ice and w a te r a r e
U s u a l l y , toward, t h e margins o f a
rock g l a c i e r a r e found c o n c e n t r i c r i d g e s , w r i n k l e s , and p i t s implying
movement by flowage o f t h e mass below t h e s u r f a c e a t some time.
However, t h e s e f e a t u r e s a r e not found in one o f t h e rock g l a c i e r s a t .
F a ir y Lake.
The flowage w ri n k le s p r e s e n t in t h e rock r u b b l e a r e found
only in t h e rock g l a c i e r on t h e n o r t h e a s t f l a n k o f Sacagawea Peak
r e l a t i v e l y high up in t h e mass and very n e a r t o t h e s o l i d rock c l i f f s
which feed th e .m a ss below.
t a l u s in l a t e summer.
Running w a te r was observed and heard, in t h e
This w a te r may have o r i g i n a t e d from me lting of
i n t e r s t i t i a l i c e , o r i t could have been w a te r esca pin g from f r a c t u r e s
and j o i n t s o f t h e bedrock.
However, due t o t h e c o a r s e n e s s o f the
d e b r i s , t h e s h e l t e r e d p o s i t i o n in t h e c i r q u e , t h e flowage w r i n k l e s ,
and t h e g en eral l o b a t e shape o f t h e t o e a r e a su gg es t t h e p o s s i b i l i t y
t h a t t h i s d e b r i s i s an i n c i p i e n t rock g l a c i e r or indeed a f u l l y
developed rock g l a c i e r .
Perhaps t h e s ourc e o f t h e w a te r may be th e
rock g l a c i e r i t s e l f ( i . e . m e l t w a t e r ) .
29
Till
T i l l i s d e fi n e d as n o n - s t r a t i f i e d , heterogeneous sediment c a r r i e d
and d e p o s i t e d by g l a c i e r i c e .
O s t e n s i b l y , i t shows no ev ide nc e of
major reworking by wind o r w a t e r .
th a t the m ajority of t i l l
However, most a u t h o r i t i e s concede
has undergone some rearra ngeme nt o f c o n s t i t ­
ue nts in re s p o n se t o changing c l i m a t e arid ph y s io g ra p h ic c o n d i t i o n s .
T i l l c o n s i s t s mainly o f m e ch a ni c al ly brok en , ab rade d, and p u l v e r i z e d
fragments o f ro ck .
The bulk o f t h e m a t e r i a l u s u a l l y c o n t a i n s c l a y ,
s i l t , or s an d, but p e b b l e s , c o b b l e s , and bould ers a r e o f t e n d i s t r i b u t e d
th ro ug ho ut t h e mass.
F l i n t (1971) makes d i s t i n c t i o n s between t i l l and
o t h e r n o n - s o r t e d heterogeneous m a t e r i a l s g e n e r a l l y known as diamictons
t h a t may be mistaken f o r t i l l .
The d e p o s i t s mapped as t i l l
definition.
in t h e B rid ge r Range f o ll o w t h i s
For t h e most p a r t , th e y a r e co nfin ed t o t h e down-cirque
p o r t i o n o f t h e v a l l e y s , and o f t e n t i l l
valley sides.
i s found on o r comprising th e
In o r n e a r t h e c i r q u e s , minor mass movements--slump,
e a r t h f l o w , mu dfl ow( ?), and a v a l a n c h i n g - - h a v e occu rr ed in t h e s te e p
v a l l e y s i d e s th u s modifying them f u r t h e r .
These movements a r e not as
p r e v a l e n t in t h e s tu d y a r e a as th e y a r e in t h e a d j a c e n t mountain ranges.
M a te r ia l t h a t resembles t i l l occurs a c r o s s t h e s o u t h e a s t headwaters o f
t h e Middle Fork o f B r a c k e t t Creek.
This m a t e r i a l has a p p a r e n t l y
"flowed" from above t o i t s p r e s e n t p o s i t i o n .
ob s e r v e d , however.
No s t r a t i f i c a t i o n was
This m a t e r i a l i s so s i m i l a r in appeara nce t o t i l l .
30
and so proximate t o d e f i n i t e t i l l t h a t i t i s here r e f e r r e d t o as
" f I o w t i l l " ( F l i n t , 1971).
Alluvium-Outwash
Alluvium and outwash d e p o s i t s a r e d i s c u s s e d t o g e t h e r and not
d i s t i n c t l y s e p a r a t e d in some p l a c e s on t h e map because o f t h e i r s i m i l a r
characteristics.
by a st re am .
Alluvium, as used in t h i s p a p e r , i s m a t e r i a l deposited
C ur re nt usage o f t h e term c o n f in e s t h e d e f i n i t i o n to t h e
s tre am d e p o s i t s o f c o m p ar at i v el y r e c e n t tim e.
Outwash, on t h e o t h e r
hand, i s a d e p o s i t d i r e c t l y downstream from t h e t e rm in a l p o r t i o n o f a
g l a c i e r o r a moraine.
Mass Wasting Deposits
L a nd s lid e d e b r i s i s i n d i c a t i v e o f r a p i d downsI ope movement o f s o i l ,
and rock.
Sharpe (1938) was one o f t h e f i r s t t o make a c o n ce rt e d
e f f o r t t o c l a s s i f y mass wast ing phenomenon.
He rec ogn iz e d f o u r broad
c a t e g o r i e s o f mass w a sti ng which he d e s i g n a t e d as slow f Towage (see
a bo ve) , r a p i d flow age, l a n d s l i d e s , and sub sid e nc e r e s p e c t i v e l y .
He
a l s o rec og ni ze d t h a t t h e r e e x i s t s a co nti nuous s e r i e s in mass w a s t i n g ,
and o f t e n t h e placement o f a s i n g l e u n i t i n t o a p a r t i c u l a r type in an.
a rb itra ry decision.
However, a good c a t e g o r i z a t i o n o f s l i d e or flow
can be made i f a s l i p o r g l i d e s u r f a c e i s r e c ogn iz e d.
A.flow has no
s l i p s u r f a c e o f any t y p e , and movement oc cur s by co ntin uous deformation.
31
S l i d e s , in c o n t r a s t , move on a s l i p s u r f a c e by f i n i t e s h e a r o ft e n
s e p a r a t i n g i n t o r e c o g n i z a b l e b lo c ks.
The f o u r broad c l a s s e s were based on t h e kind o f movement ( s l i p
o r f l o w ) , t h e r a t e o f movement (slow o r r a p i d ) and t h e n a t u r e ( s o i l o r
r o c k ) , in t h a t o r d e r , o f t h e downs!ope movement o f t h e m a t e r i a l .
In
t u r n , each o f t h e f o u r broad class es, was subdiv ide d by Sharpe as
f o ll o w s :
1) Slow f Towage ty p e s
a) creep '
b) s o i l creep
c) t a l u s creep
d) rock g l a c i e r creep
e) rock creep
f ) s o l u f l u c t i on
2) Rapid f Towage ty p e s
a) e a r t h f l o w
b) mudflow
c) d e b r i s avalan ch e
3) S l i d i n g ( L a n d s li d e s )
a) slump
b) d e b r i s - s l i d e •
c) d e b r i s - f a l l
d) rock s l i d e
e) r o c k f a l l .
'■
.
■■
..
S h a r p e ' s l i s t i n g was modified s l i g h t l y by t h e w r i t e r as f o ll o w s :
.
( I ) in c l u d i n g s o i l c r e e p , and rock creep d e p o s i t s u n d e r . a s e p a r a t e
heading o f col luv ium ; (2) s e p a r a t i n g rock g l a c i e r s and t a l u s as major
u n i t s ; (3) combining t a l u s and r o c k f a l l d e b r i s as one u n i t ; and (4) t h e
grouping o f r a p i d flowage u n i t s and some o f t h e l a n d s l i d e u n i t s i n t o a
single unit.
This modified c l a s s i f i c a t i o n s e r v e s most e a s i l y in t h e
f i e l d f o r an a c c u r a t e and complete coverage of mass wast ing ty p e s .
32
A good deal o f e f f o r t was sp en t in d i s c e r n i n g t h e d i f f e r e n t type s
and e x t e n t o f t h e d e p o s i t s in i n d i v i d u a l d ra in a g e b a s i n s ( f o r example
to d i s t i n g u i s h mass wast ing d e p o s i t s from th o s e of g l a c i a l ori gi n). .
Table I l i s t s broad c h a r a c t e r i s t i c s used as a guide t o d i s t i n g u i s h i n g
such d e p o s i t s from one a n o t h e r in t h e f i e l d .
REGIONAL GLACIAL CHRONOLOGY
The g l a c i a l time s e q u e n c e .i n t h e Rocky Mountains was f i r s t r e c og­
niz ed and enumerated by Blackwelder in 1915.
He d e l i n e a t e d and named
both t h e g l a c i a l and a s s o c i a t e d outwash d e p o s i t s (see Table 2 ) .
Several s t u d i e s a t wide ly s e p a r a t e d l o c a l i t i e s f o ll o w e d , but f u r t h e r
d e f i n i t i o n and r e f i n i n g o f t h e o r i g i n a l sequence and broad re g io n a l
.
s y n t h e s i s was not done u n t i l t h e work, o f Richmond (1948) (see Table 2)
S t i l l f u r t h e r r e s t u d y and su bsequent m o d i f i c a t i o n followe d (Richmond,
1957a, b; 1960a, b; 1964a; and 1965) r e s u l t i n g in t h e chronology used
a t t h e p r e s e n t time (s e e Table 3) .
C u r r e n t l y , t h e P l e i s t o c e n e g l a c i a t i o n o f t h e Northern Rocky
Mountains i s d i v i d e d i n t o t h r e e s t a g e s and. t h r e e i n t e r g l a c i a l s .
are:
These
I ) t h e pre-Wisconsin or pre -B u ll Lake; 2) t h e Bull Lake or
E a r l i e s t W isconsin(T ); and 3) t h e P in e d a le or Wisconsin.
Pre- Bul l Lake
Pre- Bul l Lake g l a c i a t i o n . o r g l a c i a t i o n s a r e now g e n e r a l l y con­
s i d e r e d t o be e q u i v a l e n t t o t h e pre -W is co nsi n.
Some a u t h o r i t i e s have
33
Table I .
Comparison o f f e a t u r e s o f l a n d s l i d e s and g l a c i a l d e p o s i t s
(from F l i n t and Denny, 1958; B a i l e y , 1971).
L a nd s lid e Deposits
G l a c i a l Deposits
External Form and P o s i t i o n
Massive t r a n s v e r s e r i d g e s in
headward p a r t (slump b l o c k s ) .
Sinuous t r a n s v e r s e r i d g e s in
headward p a r t ( te r m i n a l m o r a i n e s ) .
May have l o n g i t u d i n a l r i d g e s .
upstream.
Lack l o n g i t u d i n a l r i d g e s except
a t l a t e r a l margins ( l a t e r a l
moraines).
C on ce nt ri c t r a n s v e r s e r i d g e s
a r e c on fin ed t o v i c i n i t y o f t o e
and a r e no t continuous, with
marginal r i d g e s
C o n ce n tr i c t r a n s v e r s e r i d g e s
( t e rm i n a l m o ra in e s ) may be continuous
w i t h . l a t e r a l r i d g e s (moraines) and
are not r e s t r i c t e d to toe.
Possess n e a r l y s t r a i g n t t r a n s v e r s e Lack f r a c t u r e s ,
f r a c t u r e s and r a d i a l f r a c t u r e s a t
toe
Lack outwash body beyond t o e .
Commonly have ( s m a l l ) outwash
body beyond t o e .
Glaciated valley will generally
La nd sl id e v a l l e y may be
i r r e g u l a r l y blocked by l a n d s l i d e s . have open U-shaped form.
I n t e r n a l Composition
M a te ri a l d e r i v e d only from rocks
p r e s e n t on t h e l o c a l sl o p e
M at e ri a l may be d e r i v e d from any .
o r a l l rocks in g l a c i a t e d a re a s
upstream.
.
Fragments i n c lu d e very l a r g e mas­
ses o r weak bedrock and j o i n t ­
faced bo u ld e rs o f s t r o n g rock.
Fragments a r e comminuted and
mixed; in c l u d e fragm en ts with
abraded s u r f a c e s .
Loess and c a l i c h e a b s e n t , ex cept
as remnants o f p r e - s l i d e s u r f a c e .
Some c a l i c h e p r e s e n t i f p a r e n t
material is calcareous.
In p l a c e s , m a t e r i a l may be covered
with l o e s s . Rock fragments commonly
have c a l i c h e r i n d s . S o i l s with a
c a l i c h e horizon may co ver t h e t i l l .
34
Table 2.
Comparison o f C l a s s i c a l Sequence and Modified Sequence of
Wind River Mountains, Wyoming (ada pted from Richmond, 1948).
Blackwelder (1915)
Richmond (1948)
G l a c ia l Stage
Te rr a c e
. Moraine
Terrac e
Recent Ice
—
Recent Ice
flo od p l a i n
Interglacial
Interglacial
Lenore
Pi n e d a le
Stage
Circle
2b
2a
Bull Lake 2
interstadial
Bull Lake I
3b
3a
Interglacial
(canyon c u t t i n g )
Interglacial
(canyon c u t t i n g )
B uff al o Stage
2c
Interglacial
Interglacial
Bull Lake
Pi n e d a le 3
in te r s tadial
Pi n e d a le 2
interstadial
P i n e d a le I
Black Rock
Union Pass
B uff al o T i l l
4b
4a
35
Table
3
Q uaternary S tratig rap h y
Time in Y ears BP.
(A dapted from R ichm ond, 1 9 6 5 ,1 9 7 0 , B enedict, 1 9 6 8 ,1 9 7 3 ;and C u rry ,1973)
North and Central Rocky
Mountains
3 0 0 -0
Yellowstone P a rk
Gonnet Peak
Gannet P eak
4 5 0 0 -2 6 0 0
6 4 0 0 -6 0 0 0
8000
Talus
Temple Lake
Temple Lake
East Flank, Bridget Range
(this p a p e r)
Recent
Soil
Audubon
1 9 0 0 -9 5 0
Central United S ta te s
Rock G laciers?
Inlerglociol Soil
Alluvium
unnamed
Valderan
Upper
Substage
9 6 0 0 -9 0 0 0
Till
Late Stode
interstodiol
11,000- 10,000
12,000
Lake B eds
Advance 4
Two Creekon
S
5)
#
5
5
e
Middle
c
.g »
Several
o
Advances
.=
E arly Sfade
Gravel
Soil
u
^
Intra-Bull Lake Soil
Early
S tad e
8- Younger
ir
200,000
Older
2 9 0 ,0 0 0
,1
Upper Till
Rhyolite Flows
Lower Till
Rhyolite Rows
Lake Sill
Interglacial Lake Silt
Alluvial
Grovel
Altonian
I
I
Sangam onian
S tag e
Illinwan
Rhyolite
Pumice
Socagawea
Ridge
S tage
Yarmouthian Stage
Flows
Kansan
Upper PearletteIike Ash Bed
Bishop Ash
Interglacial Soil
Welded Tuff
Rhyolite
Flows
Lower Pearletle
like Ash Bed
Stage
Aftonian Stage
Welded Tuff
W ashaki Point
2
Soil ?
Advance
I
I
Lake Silt
Rhyolite
6 0 0 ,0 0 0
Advance
Lake Silt
Interglacial Soil
3
£
3
Substage
■§ 1st Episode
1 8 0 .0 0 0
1.200.000
Farmddian Substoge
Interglacial Soil
5) interglacial
Interglacial Soil
7 0 0 .0 0 0
S
%
$
3 2nd Episode
S
7 0 .0 0 0
8 0 .0 0 0
1 3 0 .0 0 0
I
Lower Till
2 5 .0 0 0
Interglacial
Advance
Till
1st Episode
interstodiol
3 2 .0 0 0
4 5 .0 0 0
5 0 .0 0 0
S o il?
Substage
H 2nd Episode
Nebraskan
Stage
I
36
t e n t a t i v e l y c o r r e l a t e d advances o f t h i s age with t h e Nebraskan, Kansan,
and I l l i n o i a n o f th e Cent ral U. S.
The age o f p re -B ull Lake d e p o s i t s
i s shown by ( I ) t h e i r deep weat he rin g p r o f i l e s , (2) o c cu r re nc e as
s h e e t l i k e masses on d i v i d e s above and beyond t h e o u t e r l i m i t s o f Bull
Lake d e p o s i t s , and (3) a s p a r s i t y or absence o f even t h e most r e s i s t a n t
b o u ld e rs .
Due t o t h e extreme d i f f i c u l t i e s , in c o r r e l a t i o n even.between a d j a ­
c e n t v a l l e y s and t o a la c k o f a b s o l u t e or r e l a t i v e age d a t e s , pre-Bull
Lake i c e advances a r e g e n e r a l l y given lo c a l names only .
Following t h e r e t r e a t o f pre -B ull Lake i c e , a warmer i n t e r v a l
p r e v a i l e d as evidenced by t h e development o f an i n t e r g l a c i a l s o i l and
by e x t e n s i v e canyon c u t t i n g in some a r e a s .
from 200. to 1000 f e e t in de pth.
The canyon c u t t i n g ranges ;
Whether source a r e a s o f i c e were
co m p le te ly i c e - f r e e or co n ta i n ed remnants o f t h e g l a c i e r s in t h e most
s h e l t e r e d . a r e a s i s unknown.
Bull Lake
The. ty pe a r e a for. Bull Lake g l a c i a t i o n i s a t Bull Lake on the
e a s t f l a n k o f t h e Wind River Mountains in Wyoming (Blackwelder, 1915).
C o r r e l a t i o n with d e p o s i t s b e li e v e d t o be o f t h i s age have been made
thr ou gh ou t t h e Rocky Mountains.
At Bull Lake, t h e t i l l forms two
t o p o g r a p h i c a l l y w e l l - d e f i n e d moraines with a s s o c i a t e d outwash p l a i n s .
- 37
...
In some a r e a s , t h r e e advances o r s t i l l s t a n d s have been recognized
(Richmond, 1964).
P i e r c e (1976) has determined by o b s i d i a n hy d ra ti o n te c h n i q u e s c a l i ­
b r a t e d by K-Ar d a t i n g methods t h a t t h e Bull Lake moraine ne ar West
Y el lowstone, Montana, f a l l s w i t h i n t h e age range g e n e r a l l y a s s o c i a t e d
wit h t h e Late pre-Wisconsin f o r t h e Rocky Mountains.
His average age
f o r t h e advance o f 140,000 y e a r s , with most measurements f a l l i n g
between 130,000 and 155,000 y e a r s , c o r r e l a t e s t h e Bull Lake end moraine
wit h t h e I l T i n o i a n G l a c i a t i o n o f t h e m id c o n ti n e n t United S t a t e s .
Undoubtedly, t h i s new evidence w i l l s t i m u l a t e r e t h i n k i n g and c o r r e ­
l a t i o n s . o f Bull Lake d e p o s i t s thro ughou t t h e Rocky Mountain Region as
well as new id e as on t h e major s o i l forming i n t e r v a l s and t h e i r
durations.
Deposits o f Bull Lake age a r e i d e n t i f i e d by t h e s e c r i t e r i a :
I)
s u r f a c e bo u ld e rs a r e more numerous than on pre-B ull Lake d e p o s i t s but
no t as abundant as P i n e d a l e ; 2) moraines a r e commonly notched very
deep ly by major s t r e a m s ; 3) moraines have well i n t e g r a t e d minor
d r a i n a g e ; 4) lower to p o g r a p h ic but h i g h e r s t r a t i g r a p h i c p o s i t i o n than
pr e -B u ll Lake morain es; 5) f r e s h e r morphological form th a n pre-Bull
Lake; .6) a l e s s e r development o f t h e w e ath ering p r o f i l e than in pre Bull Lake; 7) u s u a l l y , moraines a r e l a r g e r ; and 8) t h e y o f t e n extend
f u r t h e r outward from t h e mountain f r o n t e i t h e r pre -B u ll Lake moraines
o r P in e d a le mora ine s.
38
An i n t e r v a l o f warmer c l i m a t e followed t h e Bull Lake g l a c i a t i o n
and proceeded t h e P i n e d a l e .
Again, an i n t e r g l a c i a l s o i l (Blackwelder,
1915) developed and a r e l a t i v e l y s h o r t e r o s i o n c y cl e removed much of
t h e accumulated m a t e r i a l s from t h e v a l l e y f l o o r s and canyon mouths.
Pin e da le
The P in e d a le g l a c i a t i o n was named by Blackwelder in 1915 f o r
moraines n e a r P i n e d a l e , Wyoming, a t t h e s o u t h e a s t base o f t h e Wind
River Mountains.
At t h i s l o c a l e , P i n e d a le t i l l forms a s e r i e s of
rough, hummocky moraines d i v i s a b l e i n t o t h r e e groups.
As in t h e Bull
Lake g l a c i a t i o n s th ro ugho ut much o f t h e Rocky Mountains, a t l e a s t
t h r e e s t i l l s t a n d s o f i c e a r e re c o g n iz e d .
In some a r e a s , as many as -
e i g h t s t i l l s t a n d s have been d e l i n e a t e d (K n o ll , 1973).
Pi n e d a le
g l a c i e r s occupied t h e same v a l l e y s as th o s e o f Bull Lake b u t , in most
c a s e s , d i d n ' t extend as f a r downvalley.
P i n e d a le moraines a r e u s u a l l y f r e s h e r appea ring and c l o s e r t o t h e
so urc e a r e a s than t h o s e o f Bull Lake.
Moraines a r e commonly s t e e p ,
i r r e g u l a r , and o f t e n s m a l l e r than t h o s e o f Bull Lake g l a c i a t i o n s .
Morainal cov er i s commonly more boulde ry and l e s s d i s s e c t e d , o ft e n
c o n t a i n i n g k e t t l e s which hold w a te r a t l e a s t s e a s o n a l l y .
l a k e s a r e normal w it h P in e d a le advances.
Moraine-dammed
Primary d r a i n a g e s may notch
m or a in e s, but secondary d r a i n a g e , as i n t i m a t e d above i s p oorly i n t e ­
grated to non-ex isten t.
till
The w e ath ering p r o f i l e developed on Pin ed al e
i s l e s s mature than t h o s e found on o l d e r d e p o s i t s .
39
Following t h e r e t r e a t o f t h e l a s t s t a d e of P in e d a le i c e , g l a c i e r s
shrank d r a s t i c a l l y and, in many i n s t a n c e s , dis ap p e a re d e n t i r e l y from t h e
Rocky Mountains.
Antevs (1955) d i v i d e d t h e i n t e r v a l s t a r t i n g a t t h e end
of the l a s t g la c ia tio n u n til the present into three periods.
The
e a r l i e s t i s t h e Anathermal (10,500 t o 7000 BP), t h e middle period of
thermal maximum i s t h e A l t i t h e r m a l (7000 t o 4000 BP), a n d . t h e l a t e s t i s
t h e Medithermal (4000 t o 0 BP).
F l i n t and Deevy (1957) have named p a r t
o f t h i s time i n t e r v a l t h e Hypsithermal (9000 t o 2500 BP), whereas Bryan
and Gruhn (1964) g iv e not hin g more th a n t h e name, Neothermal, t o t h e
interval.
Rega rdles s o f t h e a p p r o p r i a t e n e s s o f a name and t h e le ng th
o f t i m e , t h e i n t e r v a l was marked by t h e poor to moderate development of
a p o s t - P i n e d a l e s o i l in t h e North Rocky Mountains.
N e o g la c ia ti o n
During t h e Neothermal i n t e r v a l , t h r e e and p o s s i b l y f o u r temporary
r e t u r n s t o g l a c i a l c o n d i t i o n s oc cu r re d ( B e n e d ic t, 1967, 1968, 1973;
P o r t e r and Denton, 1967).
These b r i e f re -adva nc es a r e c o l l e c t i v e l y
known as t h e N e o g l a c i a t i o n (Moss, 1 9 5 l b ) .
At p r e s e n t , t h r e e or perhaps
f o u r advances have been re co gnize d ( B ir ke la nd and M i l l e r , 1973; Benedict,
1967, 1968, 1973; P o r t e r and Denton, 1967).
Audubon, and Gannet Peak.
These a r e t h e Temple Lake,
The advances o f N e o g la c ia ti o n a r e names f o r
d e p o s i t s in t h e Wind River Mountains, Wyoming (Moss, 1 9 5 1 a ) , and more
r e c e n t l y f o r numerous c i r q u e s in t h e Fro nt Range o f Colorado (B e ned ic t,
40
1967, 1968, 1973; Mahaney, 1972).
Deciphering o f . N eo glacial d e p o s i t s
in t h e Northern Rockies has not y e t proceeded to any g r e a t e x t e n t and, .
/
commonly, j u s t two s t a d e s o f N e o g l a c i a t i o n a r e r e c o g n iz e d ; t h e Temple
Lake and t h e Gannet Peak (Richmond, 1965).
Oft en , an e a r l y s t a d e , now
r e f e r r e d t o as Temple Lake in some a r e a s and " l a t e s t P in e da le " in
o t h e r s , has been found.
way.
A d e bat e as t o t h e age o f t h i s s t a d e i s under­
The q u e s t i o n i s whether t o a s s i g n t h e moraine t o . p r e - A l t i t h e r m a l ,
t h e f i r s t stade. o f t h e Neoglacial sequence, or p r e - H y p s i t h e r m a l , an
even l a t e r s t a d e o f P in e d a le (Mahaney, 1972; B irk el an d and M i l l e r , 1973;
Curry, 1973).
Rough and bouldery moraines o f Neoglacial age a r e o f t e n c l o s e t o .
or within cirques.
However, some o f t h e o u t e r moraines a r e ve get a te d
by tu n d r a h e r b s , s h r u b s , and f i r t r e e s (B e n ed ic t, 1973).
Deposits o f
N e o g l a c i a t i o n commonly in c lu d e p r o - t a l u s r a m p a r t s , l o b a t e - , tongueand d i g i t a t e - s h a p e d rock g l a c i e r s , a c t i v e and i n a c t i v e t a l u s , and ro c kf a l l debris.
SURFICIAL GEOLOGY OF THE MAP AREA
Summary
In t h e d i s c u s s i o n which f o l l o w s , t h e s u r f i c i a l d e p o s i t s w il l be
d e s c r i b e d by d r a in a g e b a s i n s r a t h e r th a n by c o n s i d e r i n g each s u r f i c i a l
u n i t th r ou ghou t t h e e n t i r e stu dy a r e a . .
41
The g en eral s e q u e n c e . o f g l a c i a t i o n s o f t h e n o r t h e a s t f l a n k of t h e
B rid ge r Range i s based on r e l a t i o n s e s t a b l i s h e d by t h e w r i t e r in th e
F a ir y Creek d ra in a g e ( d i s c u s s e d below).
In summary, t h e sequence
i n c lu d e s f o u r d i s t i n c t advances and r e t r e a t s o f i c e and, accompanying
and f o ll o w i n g d e g l a c i a t i o n , t a l u s ac c um ul a tio n, rock g l a c i e r fo r m a ti o n ,
and l a n d s l i d i n g .
The o l d e s t advance. Advance I (Bull Lake?), was th e
most e x t e n s i v e and t h e l a r g e s t .
The second advance. Advance 2 (Rinedale
I ? ) , was almost as l a r g e and as long.
Advance I , and t h e t i l l
for mer.
I t s outwash b u r i e d p o r t i o n s o f
i s n e a r l y as high on t h e v a l l e y w a ll s as. th e
The. t h i r d advance. Advance 3 (Rine dal e I I ? ) extended only a
s h o r t d i s t a n c e beyond t h e c i r q u e s and i s r e p r e s e n t e d by t h e moraine
damming F a i r y a n d .E l$ Lakes.
and l e a s t e x t e n s i v e .
Advance 4 (Rinedale I I I ? ) was t h e s h o r t e s t
This advance i s r e p r e s e n t e d by moraines and
morainal remnants a t o r j u s t beyond t h e c i r q u e t h r e s h o l d s upstream from
F a i r y and E l f Lakes.
Presumably, accompanying o r fo ll o w i n g d e g l a c i a t i o n , t a l u s accumu­
l a t i o n and rock g l a c i e r formation began.
Lan dsl id in g which include d
d is pl a ce m e nt o f t i l l a l s o accompanied t h i s phase.
In a l l t h e d r a i n a g e s ,
t h e l a n d s l i d i n g and t a l u s accumulation have co ntinued u n t i l t h e p r e s e n t
time.
' \
The sequence i s t h e same as t h a t o f t h e Fa ir y Creek Basin f o r a l l
t h e d r a in a g e b a s i n s in t h e map a r e a . t o t h e south o f F a i r y Creek with
a minor e x c e p t i o n .
The e x c e p ti o n i s t h a t only t h e f i r s t t h r e e r e a d i l y
42
d i s t i n g u i s h a b l e s t a d e s a r e found in t h e o t h e r d r a i n a g e s . . The fo u r t h
s t a d e , r e s t r i c t e d t o c i r q u e s , i s not p r e s e n t in t h e o t h e r d r a i n a g e s . .
The b a s i n s which w i l l be d i s c u s s e d a r e t h e F a ir y Creek, Cache Creek,
and North Fork, Middle Fork, and South Fork o f B r a c k e t t Creek.
F a i r y Creek Basin
"
Introduction.
T h e . g l a c i a l d e p o s i t s o f t h e s tu dy a r e a as well as
t h e e a s t f l a n k o f t h e B ri dg e r Range a r e b e s t developed and d is p la y e d
in t h i s d r a in a g e b a s i n .
T h e r e f o r e , t h i s p o r t i o n o f t h e s tu dy a re a was
deemed most c r u c i a l in d e c ip h e r in g t h e p ro ba ble sequence o f g l a c i a l
events.
.
The c r e s t a l a re a o f t h e Range above F a ir y Greek i s notched by t h e
most s p e c t a c u l a r c i r q u e s in t h e B ri d g e rs (see f i g u r e 5 ) .
.
This p a r t of
t h e Range i s t h e b r o a d e s t as well as g e n e r a l l y , t h e h i g h e s t in e l e v a t i o n .
The h i g h e r e l e v a t i o n s and broade r landmass o f f e r e d a much l a r g e r c a t c h ­
ment a r e a f o r f a l l i n g and d r i f t i n g snow.
These f a c t o r s , in p a r t , a r e
r e s p o n s i b l e f o r t h e profound development o f c i r q u e s in t h e a re a compared
t o t h e l e s s e r development o f e q u i v a l e n t segments t o t h e s o u th .
The .
pr e s en c e o f t h e n o r t h w e s t - s o u t h e a s t t r e n d i n g Cross Range F a u l t (see
f i g u r e 4) a l s o i n f l u e n c e d t h e l o c a t i o n and development o f t h e p r e ­
g l a c i a l and g l a c i a l v a l l e y o f F a ir y Creek.
The F a u l t Zone, obviously
a zone o f weakness, was e x p l o i t e d and e n l a r g e d . b y both t h e stream and.
even more so by i c e which s u b s e q u e n tl y occupied t h e v a l l e y . .
The
43
Fig ur e 5. Panorama o f c i r q u e s o f t h e s tu dy a r e a . Ross Pass i s the
deep notch a t f a r l e f t o f photo. Sacagawea Peak and t h e F a ir y Lake
c i r q u e a r e in t h e c e n t e r o f photo.
44
h y p o th e s i s o f e x p l o i t a t i o n o f a p r e e x i s t i n g weakened zone may p a r t l y
acc oun t f o r t h e huge s i z e o f t h e F a i r y Creek te rm ina l morain es.
In
a d d i t i o n , a n o t h e r im po rt an t f a c t o r i s t h e ty pe of bedrock t r a v e r s e d by
t h e i c e ; t h e e a s i l y e r o d a b l e s t r a t a o f Mesozoic Age which were removed
in a d i r e c t i o n g e n e r a l l y o b l i q u e t o t h e s t r i k e o f t h e beds.
Lower P a r t o f F a ir y Creek B a s i n .
The lower p a r t o f t h e Fa ir y Creek
ba si n i s covered by an assemblage o f a ll uvium -ou tw as h, moraines o f two
advanc es, and bedrock r i d g e s surrounded by e x t e n s i v e a r e a s o f colluvium
and residuum.
The e a s t e r n p a r t o f t h e F a ir y Creek b a s i n , l i k e t h e a r e a s t o t h e
s o u t h , i s u n d e r l a i n by bedrock o f t h e Li vi n g st o n Grdup ( s e e P l a t e I ) .
S u r f i c i a l d e p o s i t s t h e r e c o n s i s t more o f colluvium and residuum than
any o t h e r m a t e r i a l .
In t h e extreme n o r t h e a s t e r n p a r t ( s e c . 17, T 2 N,
R 7 E ) , however, i s an e x t e n s i v e a re a o f outwash.
This outwash can be
t r a c e d upstream t o t h e termi nus o f t h e F a i r y Creek moraine in SW
Sec. 18, T 2 N, R 7 E and in t h e SE %, T 2 N, R 6 E and through th e
o u t e r moraine t o a second terminus ap pro xi ma te ly t w o - t h i r d s o f a mile
upstream from t h e f i r s t .
The outwash downstream from t h e o u t e r moraine
i s most l i k e l y a composite from t h e f i r s t and second advances.
Alluvium
and outwash oc cur in t h e North Branch o f Cache Creek in Secs. 19 and 20,
T 2 N, R 7 E.
advance.
This outwash i s th oug ht t o have o r i g i n a t e d from th e f i r s t
45
A l a r g e p o r t i o n o f t h e o l d e r moraine i s bu rie d under p i t t e d outwash
o f t h e second advance.
The p i t t e d outwash l i e s in t h e e a s t - c e n t r a l p a r t
o f Sec. 13, T 2 N, R 6 E and th e w e s t - c e n t r a l p a r t o f Sec. 18, T 2 N,
R 7 E.
I t d i s p l a y s very hummocky topography u n d e r l a i n by b o u l d e r s ,
.
c o b b l e s , and pebbles o f l i m e s t o n e , s a n d s t o n e , some s i l t s t o n e , and an
e xtr em el y small f r a c t i o n o f s h a l e .
Some f i l l i n g o f i n t e r s t i c e s by
f i n e r m a t e r i a l ( e o l i a n ? ) i s i n d i c a t e d by t h e smoothed o u t , f a i n t l y
subdued n a t u r e o f t h e s u r f a c e .
.
-
The moraines r e f e r r e d to above dominate t h e middle reach es of
F a i r y Creek.
They make up t h e topography o f h a l f of Secs. 18 and 19,
T 2 N, R 7 E and Secs. 13 and 24, T 2 N, R 6 E.
.
The oute rmo st moraine was d e p o s i t e d by th e o l d e s t i c e advance
rec ord ed h e r e .
The moraine may c o r r e l a t e with t h e Bull Lake moraine
of t h e c l a s s i c a l Rocky Mountain sequence.
This i n t e r p r e t a t i o n is. based
on to p o g r a p h ic and s t r a t i g r a p h i c p o s i t i o n as well as to pogr a phic form.
The moraine r e c o r d s t h e f u r t h e s t advance o f i c e .
I t underlies deposits
of t h e second advance and p o s se ss e s, fewer k e t t l e s , smoother and more ,
rounded s l o p e s , s t e e p e r s l o p e s , and a g e n e r a l l y l a r g e r form.
.
a l s o has fewer bou ld e rs on i t s s u r f a c e .
The moraine
!
Those p r e s e n t a r e u s u a l l y
l a r g e , rounded blocks o f Madison Limestone.
O c c a s i o n a l l y , s m a l le r
blocks o f san ds to ne a r e encou nte red.
The second moraine i s th ou gh t t o be younger by v i r t u e o f i t s
p o s i t i o n behind and o v e r la p p in g t h e f a r t h e s t advance.
This advance i s
•
46
b e l i e v e d t o t e n t a t i v e l y c o r r e l a t e with t h e e a r l i e s t P in e d a le or Pi ne da le
I . o f t h e , c l a s s i c a l sequence.
The c o r r e l a t i o n i s made on t h e b a s i s of a
g r e a t e r amount and b e t t e r d e fi n e d knob and k e t t l e top ography, an
abundance of l im e s to n e and sand st one bou ld e rs on t h e s u r f a c e , and a
s m a l l e r volume o f m a t e r i a l , pos ses se d by t h i s moraine.
A br o a d , f a i r l y f l a t - b o t t o m e d tro ugh s e p a r a t e s Advance I from
Advance 2.
The tr ou gh i s a m e lt w a te r c h a n n e l .
I t i s well developed
and conspicuous on a i r photos as well as on t h e ground.
The v a r i o u s ages and advances o f i c e in t h e Rocky Mountain Region
and in t h e m i d - c o n t i n e n t r e g io n have been d i f f e r e n t i a t e d using r e l a t i v e
s o i l development o f t i l l o f t h e v a r io u s moraines (Richmond, 1948, 1957a,
b, 1962a, b, 1964a, 1965; B i r k e l and, 1972; G i l e s , 1970).
In t h e Fa ir y
Creek b a s i n , no a p p a r e n t o r s i g n i f i c a n t d i f f e r e n c e s were noted in s o i l
development between Advance I and Advance 2 as based on a f i e l d exami­
n a t i o n o f t h e r e l a t i v e l y few a v a i l a b l e exposures o f t h e s o i l p r o f i l e
of the t i l l .
Fur therm ore, no s i g n i f i c a n t d i f f e r e n c e s were noted in
s o i l c o l o r , de pth o f h o r i z o n s , and number o f horiz ons p r e s e n t .
Thus,
t h e s o i l s erved t o i d e n t i f y t i l l and moraines and t o help d i f f e r e n t i a t e
t i l l from o t h e r d ia m ic to n s .
However, i t did not a p p r e c i a b l y a id in
de te rm in in g t h e r e l a t i v e ages o f advances.
A s m a l l , i n t e r m i t t e n t la k e occu pie s a p o s i t i o n ne ar t h e j u n c t i o n
o f Secs. 13 and 24, T 2 N, R 6 E and Sec s. 18 and 19, T 2 N, R 7 E
(see fig u r e 6).
D i r e c t l y west o f t h e l a k e . F a ir y Creek i s i n c i s e d
47
Figure 6. I n t e r m i t t e n t la ke in t h e te rm in a l moraine o f Advance II
(P i n e d a l e I ? ) ne ar t h e j u n c t i o n o f Secs. 13 and 24, T 2 N, R 6 E and
Secs. 18 and 19, I 2 N, R 7 E.
48
a pp ro xi m a te ly 80 t o HO f e e t i n t o t h e moraine o f Advance 2.
Bedrock i s
not exposed in t h e w a ll s o f t h e stre am c u t nor a t stream l e v e l .
a t t h i s l o c a l i t y . F a i r y Creek i s e n t i r e l y in t i l l .
Thus,
No l o c a t i o n was
found where t h e e n t i r e t h i c k n e s s o f t i l l o v e r l y i n g bedrock i s exposed.
T h e r e f o r e , no e s t i m a t e o f t h i c k n e s s o f t h e d r i f t could be made f o r
Advances I and 2.
McMannis (1.955) p o s t u l a t e d t h a t t i l l t h i c k n e s s
ranges from a few t o more than 250 f e e t on t h e e a s t s i d e o f t h e Range.
Middle P a r t o f F a ir y Creek B a s i n .
The middle r e a ch e s o f t h e Fa ir y
Creek ba si n a r e dominated by t i l l from Advances I and 2, a l a r g e area
of l a n d s l i d e d e b r i s and a bedrock r i d g e on t h e no rt h w e s t s i d e of t h e
v a l l e y covered by colluvium and residuum.
D i r e c t l y a d j a c e n t t o t h e upstream from t h e l a k e , a moderately
l e v e l a r e a occu pi es t h e v a l l e y f l o o r from t h e la ke al ong t h e s o u t h ­
e a s t e r n bank o f F a i r y Creek t o j u s t above t h e p o i n t where Fa ir y Creek
i s c ro s se d by t h e F a ir y Creek Road (NE %, Sec. 24, T 2 N, R 6 E).
This f l a t s t r e t c h b ear s comment as i t i s t h e only such s u r f a c e in th e
s tu d y a r e a .
I t could p o s s i b l y be a t e r r a c e remnant, but no o t h e r such
remnants e x i s t in t h e e n t i r e v a l l e y .
I t could a l s o be t h e former
channel o f a stream ( F a i r y Creek?) t h a t d r a in e d a g l a c i e r occupying an
u p v a ll e y p o s i t i o n , hence an.outwash p l a i n remnant.
This i n f e r e n c e i s
based on t h e p resen ce o f a m e lt w a te r channel t o t h e e a s t o f t h e lake
which connects w it h t h e p l a i n .
A nc es tr a l F a i r y Creek was d i v e r t e d no rt h .
o f and around t h e p l a i n t o i t s p r e s e n t c ours e a t some p o i n t upstream.
49
W
9000
8000
E
Bridger Divide
Croee Range Fault
Fairy Creek
7000
F airy C reek Terminus
6000
6000
Figure 7. Long it udi nal P r o f i l e of Fa iry Creek Valley ( V e r t i c a l
e x a g g e ra ti o n i s 2.5X) (See P l a t e I f o r an e x p l a n a t i o n o f th e
s ym bo ls ).
50
The so u th er n s i d e o f t h e F a ir y Creek v a l l e y i s formed by t h e huge,
composite l a t e r a l moraine, o f Advances I and 2 of t h e F a i r y Creek i c e
(see f i g u r e 8 ) .
In t h e middle and lower p o r t i o n s o f t h e v a l l e y , t h e
l a t e r a l moraine r e t a i n s a c l a s s i c shape.
In i t s upper p o r t i o n s in t h e
v i c i n i t y o f F a i r y Lake, however, t h e moraine i s s i g n i c a n t l y modified by
landslides.
These a r e by f a r t h e l a r g e s t in t h e stu dy a r e a .
The most
r e c e n t i s e a s i l y v i s i b l e from t h e F a i r y Lake Road a c r o s s t h e v a l l e y and
o c cu r re d in 1960 ( J . Montaigne, o ra l Communication, 1975).
l o c a t e d in t h e SW %, Sec, 23, T 2 N, R 6 E;
en tirely of t i l l .
I t is
This l a n d s l i d e i s composed
I t appea rs on a i r photos and in t h e f i e l d t h a t t h i s
l a n d s l i d e oc cu rr ed in a much l a r g e r , o l d e r l a n d s l i d e .
The o l d e r la n d ­
s l i d e oc cu pie s most o f t h e so u th er n q u a r t e r o f Sec. 23, making up. t h e
s i d e and f l o o r o f t h e v a l l e y (s e e f i g u r e 8 ) .
Adjacent t o and west of
t h e 1960 l a n d s l i d e i s a .s e c ond area, o f l a n d s l i d e . d e b r i s .
Both bedrock
and t h e o v e r l y i n g t i l l a r e involved in t h i s a r e a o f s l i d i n g .
The n o r th s i d e o f F a ir y Creek v a l l e y from t h e moraine o f Advance 2
t o t h e moraine o f Advance 3 c o n s i s t s o f t i l l , bedrock, and t i l l covered
bedrock.
Subdued, n e a r l y for m le s s t i l l o f t h e n o r t h e r n l a t e r a l moraine
forms t h e lower p a r t o f t h e v a l l e y from t h e terminus o f Advance 2 to
t h e SW
Sec. 24, T 2.N, R 6 E.
In t h i s v i c i n i t y , t h e v a l l e y s i d e
c o n s i s t s o f b e d r o c k i co lluvium and some residuum.
Northwest from t h i s
l o c a l e t o t h e F a i r y Lake t e rm in a l m ora ine , bedrock, colluvium and one
l a n d s l i d e make up t h e v a l l e y s i d e .
The f l o o r of t h e v a l l e y i s veneered
51
Figure 8. F a ir y Creek Terminal Complex viewed Eastward from
Sacagawea Peak. La nds lid e o f 1960 i s v i s i b l e in t h e lower r i g h t
c o r n e r o f photo. The bedrock r i d g e from t h e c e n t e r o f t h e photo to
t h e middle l e f t i s composed of Upper P a le o z o i c and J u r a s s i c roc ks .
52
N
S
Cross R ange Fault
Z z
Fairy Creek
8000
8000
7000
7000
Figure 9. Tr a n s v e rs e P r o f i l e of Upper P o r ti o n of F a ir y Creek in
Ea s te rn
Sec. 22, T 2 N, R 6 E ( V e r t i c a l e x a g g e ra ti o n i s 2.5X)
(See P l a t e I f o r an e x p l a n a t i o n o f t h e symbols).
N
S
Foiry Creek
7000
6000
6000
Figu re 10. Tr an s ve rs e P r o f i l e of Middle P o r ti o n o f F a i r y Creek NE k,
Sec. 24, T 2 N, R 6 E ( V e r t i c a l e x a g g e r a t i o n i s 2.5X) (See P l a t e I f o r
an e x p l a n a t i o n of th e sy mbols).
53
by a t h i n l a y e r o f ground moraine.
_
Th e. co ver o f t i l l
is postulated to
be t h i n he re because F a i r y Creek flows a c r o s s bedrock u n i t s from the
E l l i s Group t o t h e Eagle Formation in t h e SE h, Sec. 23, T 2 N, R 6 E.
The t i l l c o v e r , on t h e o t h e r hand, may be q u i t e t h i c k , and th e bedrock
may be merely a p a r t i a l l y b u r ie d bedrock high which has been exposed
only along F a i r y Creek.
South o f t h i s s e c t i o n , t h e s u r f i c i a l m a t e r i a l ,
i s t h e t o e o f t h e o l d e r l a n d s l i d e mentioned a b o v e . .
Upper P a r t o f F a ir y Creek B a s i n .
The upper p a r t o f Fa ir y Creek
b a s in c o n t a i n s t h e l a r g e c i r q u e s , t a l u s , rock g l a c i e r s , and both
morainal remnants and moraines.
The F a ir y Lake te rm in a l moraine forms a nort hw es t tr e n d i n g r id g e
a c r o s s this v a l l e y o f F a ir y Creek.
I t i s continu ous from t h e south wall
o f t h e v a l l e y t o t h e n o r th wall o f a c i r q u e l o c a t e d sou th o f Hardscrabble
Peak.
I t dams two o f t h e f o u r permanent la k e s in t h e B ri d g e r Range.
la ke s e x i s t f o r two r e a s o n s :
I ) t h e moraine holds back t h e w a te r ; and
2) th e la kes are. u n d e r l a i n by t h e Morrison Formation.
The Morrison •
Formation i s composed o f s h a l e and mudstone with some sa n d s to n e .
The
s h a l e and mudstone, when p u l v e r i z e d as t h e y would have been by i c e ,
became impermeable and made-a good " s e a l e r "
f o r t h e la k e beds.
Thus,
t h e w a te r was r e t a i n e d in t h e form o f F a i r y and E l f Lakes.
The moraine has a mound shape but la c k s knob and k e t t l e s u r f a c e .
expression.
The
I t i s p r o f u s e l y covered w it h bou ld e rs o f l i m e s t o n e .
54
sandstone, s i . l t s t o n e , and some q u a r t z i t e .
This moraine may c o r r e l a t e
with t h e P in e d a le II of. t h e c l a s s i c a l Rocky Mountain sequence.
Portions
o f t h e moraine behind and nor th w es t o f E l f Lake a r e cored by bedrock o f
the. S w if t Formation o f t h e E l l i s Group.
Outcrops behind E l f Lake a r e
rounded and smoothed where not covered by t i l l
(s e e f i g u r e 11).
These
o u tc ro p s a l s o c o n t a i n gouges, s t r i a t i o n s , and some g l a c i a l p o l i s h (see
f i g u r e 12);
The s t r i a t i o n s i n d i c a t e a t l e a s t two s e p a r a t e d i r e c t i o n s
o f i c e flow , but o t h e r s t r i a t i o n s a r e no t so s t r o n g l y o r i e n t e d .
This
complex assemblage i s no t s u r p r i s i n g when one r e c o g n iz e s t h a t t h i s was
an a r e a o f very a c t i v e i n t e r p l a y between t h e i c e lobe s from th e c i r q u e s
d i r e c t l y upvalley.
E l f Lake empties i n t o a stream which has c u t a narrow gorge
through t h e moraine and i n t o t h e bedrock.
.
Approximately 6 t o 8 f e e t o f
t i l l o v e r l i e s t h e bedrock a t t h i s p o i n t (s e e f i g u r e 13).
The s o i l
developed here i s c o n s id e r e d t y p i c a l f o r t h e moraine.
Behind t h e moraine and in f r o n t o f t h e s t e e p , a b r u p t fa c e of t h e
mountains p ro pe r i s an a r e a c h a r a c t e r i z e d , by t a l u s s l o p e s or p r o - t a l u s
r a m p a r t s , rock b o s s e s , ground moraine, and some f o s s i l rock g l a c i e r s .
D i r e c t l y west o f F a i r y Lake a r e a number o f bedrock o u t c r o p s .
These
i n c lu d e small exposures of t h e E l l i s Group, t h e Quadrant Formation, the
Big Snowy Group, and t h e Amsden Formation.
The Quadrant s t i l l r e t a i n s
well p re s e rv e d g l a c i a l p o l i s h , s t r i a t i o n s , and s om e. l ar ge f l u t i n g s and
grooves.
In t h e same v i c i n i t y n e a r t h e a x i s of t h e v a l l e y a r e some
55
Figure 11. G l a c i a l l y smoothed and rounded outc rop of t h e Swift
Formation west o f E l f Lake.
56
Figure 12.
Figure 11.
G l a c ia l s t r i a t i o n s in Sw ift Sandstone i l l u s t r a t e d in
Lens cover i s 2% inc hes in dia m e te r .
57
Figure 13. Exposure of t i l l o f t h e Fa ir y Lake te rm ina l moraine in
stream c u t between E l f and Fa ir y Lakes. Note extreme a n g u l a r i t y of
b o u ld e r s .
58
l a r g e ou tc ro p s o f Madison l i m e s t o n e .
These rocks show t h e i n f l u e n c e s o f
s o l u t i o n in t h e form o f l a p i e s , s o l u t i o n p i t s , and a g e n e r a l l y rough,
hackly s u r f a c e .
I f t h e y were e ve r p o l i s h e d or s t r i a t e d , a l l evidence
has s i n c e been e r a d i c a t e d .
They a r e , however, g e n e r a l l y smoothed and.
rounded showing some e f f e c t s o f g l a c i a t i o n .
The c i r q u e forming t h e s t e e p n o r t h c l i f f of Sacagawea Peak i s th e
l a r g e s t on t h e e a s t s i d e of t h e B ri dge r Range (see f i g u r e 14).
q u a r t e r s o f t h e c i r q u e a r e c u t in t h e Madison Group.
Three
Limestone o f t h e
Mission Canyon Formation forms t h e t h r e s h o l d when not covered by t a l u s
or t i l l .
The c r e s t o f t h e Range i s composed o f Upper Cambrian and ..
Devonian r o c k s .
The Lodgepole Formation comprises t h e i n t e r v e n i n g ro ck .
A major s h a r e o f ru b b l e forming t h e t a l u s which p a r t i a l l y f i l l s t h e
c i r q u e i s c o n t r i b u t e d by Lodgepole.
A l a r g e , c o a r s e , and a n g u l a r blocky d e p o s i t , which q u i t e p o s s i b l y
i s t h e l a t e r a l morainal remnant o f t h e c i r q u e - c o n f i n e d g l a c i e r , e x i s t
along t h e n o r t h wall o f t h e c i r q u e in t h e SE %, SW %, Sec. 22, T 2 N,
R 6 E.
I f a l a t e r a l moraine did e x i s t on t h e sou th s i d e o f t h e c i r q u e ,
i t i s now c om pl e te ly obscured by t a l u s from t h e south w a l l .
The
t e rm in a l p o r t i o n s o f t h i s s t a d e a r e found in two small p a tc h e s in t h e
SW %, SE
Sec. 22, T 2 N, R 6 E a l s o .
I f t h e s e d e p o s i t s a r e indeed
t i l l remn an ts, a f o u r t h advance o r a Rine dale I I I c o r r e l a t i o n is
indicated.
59
Figure 14. F a ir y Lake c i r q u e . Sacagawea Peak i s to t h e l e f t . Note
t h e l a r g e bos ses of Mission Canyon Limestone near t h e c i r q u e t h r e s h o l d .
60
An i n t e r m i t t e n t stream d r a i n s t h e e n t i r e c i r q u e and s e p a r a t e s the
two te rm in a l remnants.
deep t r e n c h .
I t flows through t h e remnants in a 15 to 20 f o o t
F urt her .dow nst ream , i t i s i n c i s e d i n t o t h e bedrock.
The s o u t h e a s t e r n m o s t remnant o f t h e te rm in a l p o r t i o n s o f th e
moraine he lp s t o form t h e t h r e s h o l d o f t h e c i r q u e .
Like t h e l a t e r a l ,
i t i s composed o f c o a r s e and a n g u l a r blocks o f li m e s t o n e .
The remnant
i s p i l e d behind and onto a bedrock boss o f Mission Canyon Limestone..
Talus from t h e sou th wall o f t h e c i r q u e p a r t i a l l y b u r i e s t h e moraine.
The second remnant o f t h e te rm in us i s ap pro xim a te ly midway between
t h e n o r th wall l a t e r a l moraine remnant and t h e so u th er n te rm in a l
remnant.
southern.
The i n t e r m i t t e n t stream s e p a r a t e s t h e mid-segment from th e
I t i s s e p a r a t e d from t h e n o r t h wall l a t e r a l moraine remnant
by a low s p o t which may be an old m e lt w a te r escape r o u t e .
This morainal
remnant i s a l s o p i l e d behind and onto a boss o f .M is s io n Canyon Limestone.
Small rock g l a c i e r s (?) a r e found, on t h e n o r t h e a s t and southwest,
sides of the cirque.
I d e n t i f i c a t i o n o f rock g l a c i e r s i s based on a i r
photo evidenc e and f i e l d e xam in atio n.
During t h e f i e l d exami nat ion ,
w a te r was heard flowing in t h e t a l u s and seen t r i c k l i n g from t h e f r o n t s
of the d ep o sits.
Flowage l i n e s and w r in k le s were observed in the upper
p a r t o f t h e t a l u s o f t h e southwest s i d e rock g l a c i e r .
The rock g l a c i e r
o f t h e s o u t h e a s t s i d e b e s t conforms t o t h e d e s c r i p t i o n o f t h e tongue­
shaped rock g l a c i e r o f Madole (1972) and White (1975).
I t i s much
lo n g e r than broad and sp re ad s l a t e r a l l y when i t re a c h e s t h e v a l l e y
61
floor.
The rock g l a c i e r o f t h e n o r t h e a s t s i d e i s th ought to be com­
p r i s e d , in p a r t , of t h e l a t e r a l moraine.
This m a t e r i a l was simply
i n c o r p o r a t e d w i t h o u t s i g n i f i c a n t reworking or morphological change.
The rock g l a c i e r b e s t f i t s th e d e f i n i t i o n o f t h e Tobate rock g l a c i e r
o f Madole (T972) and White (1975).
There i s a d i s t i n c t p o s s i b i l i t y ,
t h a t t h e rock g l a c i e r o r i g i n a t e d from g l a c i e r i c e being covered and
p re s e rv e d by t a l u s from t h e n o r t h w a l l .
massive d e p o s i t w ith a s t e e p . f r o n t .
in t h e toe p o r t i o n s .
This rock g l a c i e r i s a l a r g e ,
No flowage, however, was observed
D i r e c t l y behind t h e l a r g e f r o n t r i d g e i s an
e l o n g a t e furrow s i m i l a r t o th o s e d e s c r i b e d by Madole (1972). and White
(1975).
The f l o o r o f F a i r y Lake c i r q u e i s l i t t e r e d w ith rock d e b r i s
d i s t i n c t in p o s i t i o n and form from t h e t a l u s s lo p e s and morainal
remnants.
Much o f t h e m a t e r i a l a ppea rs t o have been moved a s h o r t .
d i s t a n c e by w a te r .
An a l p i n e a l l u v i a l fan formed by r e p e a t e d mudflows
from t h e Three Forks and Sappington Formations may e x p l a i n t h e d e p o s i t .
The Three Forks and Sappington Formations occupy t h e nort hw e s t c o r n e r .
of the cirque.
The d e p o s i t heads in t h i s a re a Tending f u r t h e r s uppor t
to the theory.
The p r e s e n t i n t e r m i t t e n t s tr e am , flowing a c r o s s the
f a n , has become i n c i s e d 2 t o 3 f e e t i n t o t h e fan i t s e l f .
The c r e s t a l s e c t i o n s o f both t h e backbone of t h e Range and the
i n d i v i d u a l peaks and s purs s e p a r a t i n g c i r q u e s of t h e a r e a a r e c h a r a c ­
t e r i z e d by rock r u b b l e .
The blocks most l i k e l y o r i g i n a t e d from f r o s t
62
s h a t t e r i n g o f t h e bedrock.
Some o f t h e s e s e c t i o n s a r e connected with,
c h u te s f e e d in g t a l u s p i l e s and s lo p e s o f t h e c i r q u e s below. ..
North o f t h e F a i r y Lake c i r q u e and south of t h e H ardscr ab ble Peak
c i r q u e a r e two well developed c i r q u e s and a t h i r d l e s s well developed
one.
This c i r q u e (s e e f i g u r e 15) i s bro a d, not deeply c u t , and has no
a p p r e c i a b l y developed headwall.
than t h e v a l l e y p ro p e r.
The t h r e s h o l d i s c o n s i d e r a b l y . h i g h e r
Huge blocks o f t a l u s of t h e Mission Canyon
Formation, a t t h e t h r e s h o l d s p i l l over i n t o t h e v a l l e y in t h e a r e a behind
E l f Lake.
They form a t a l u s s lo p e t h a t descends t o t h e v a l l e y f l o o r
where i t sp re ad s l a t e r a l l y in both n o r th and south d i r e c t i o n s .
now c o n s i d e r a b l y t r e e covered in t h e lower p o r t i o n s .
I t is
This t a l u s i s the
l a r g e s t along t h e f a c e o f t h e mountains in t h e F a ir y Lake a r e a .
A lob e -s hap e d mass of hummocky, ex tre m el y a n g u l a r , bodl d e r l y
blocks in t h e NW %, Sec. 22, T 2 N, R 6 E i s bu ri e d by t h e n o r t h e a s t
segment o f t h e t a l u s s l o p e mentioned above.
The d e p o s i t i s c onst ru ed
t o be an end moraine which i s a c o r r e l a t i v e with t h o s e morainal remnants
o f t h e F a i r y Creek c i r q u e .
The i c e which d e p o s i t e d t h e moraine flowed
only a s h o r t d i s t a n c e from t h e c i r q u e behind i t .
L a t e r a l s o f the
moraine extend t o t h e w a l l s o f t h e c i r q u e where they a r e bu ri e d beneath
talus.
The c i r q u e i t s e l f i s f i l l i n g t h e t a l u s .
The floow i s buried t o
an unknown de p th .
The l a s t major c i r q u e t o c o n t r i b u t e t o t h e F a ir y Creek lobe i s
a d j a c e n t t o but south o f H ardsc rabble Peak ( s e e f i g u r e 16).
As i t s
63
Figu re 15. At c e n t e r s k y l i n e o l d e r (?) c i r q u e o f F a ir y Lake which was
s u c c e s s i v e l y occupied by Bull Lake and Pi n e d a le i c e .
I
I
t
65
ne ig hbo rs t o th e s o u t h , t h i s c i r q u e i s a l s o f i l l i n g w ith t a l u s .
A
small i c e tongue o r i g i n a t e d in t h e c i r q u e during t h e l a s t s t a d e of
glaciation.
I t pushed ou t and down t h e v a l l e y u n t i l i t met t h e i c e
from i t s s i s t e r c i r q u e to t h e s o u th . . Unable t o s h o u l d e r a s i d e t h e . i c e
o r flow around i t , t h e n o r t h e r n lobe was blocked and f o r c e d t o dump
i t s load in an e l o n g a t e moraine.
and form t o i t s so u th er n ne ig hb or .
•
This moraine i s s i m i l a r in composition
The moraine c o n s i s t s o f c o a r s e ,
a n g u l a r blocks o f lim e s to n e p i l e d i n t o knobs and k e t t l e s .
I t corre­
l a t e s wit h both i t s so u th er n c o u n t e r p a r t and t h e morainal remnants o f
t h e F a ir y Lake c i r q u e .
An i c e marginal channel along t h e o u t s i d e o f t h e north w es ter n
l a t e r a l moraine i s now occupied by.an i n t e r m i t t e n t st re am .
In th e SW %,
Sec. 15, T 2 N, R 6, E i s a s m a l l , l o b a t e - s h a p e d d e p o s i t o f l a r g e , angular
blocks o f li m e s t o n e . . The d e p o s i t i s b e l i e v e d t o be an i n a c t i v e rock
glacier.
Cache Creek Basin
The Cache Creek d ra in a g e ab u ts t h e F a i r y Creek d r a i n a g e on th e
s o u th .
I t i s probably t h e most con fus ing from a s u r f i c i a l ge olo gic
s t a n d p o i n t , e s p e c i a l l y in t h e c e n t r a l p o r t i o n . ,The headwaters area
and c e n t r a l rea ch e s o f t h e ba sin were covered by i c e which o r i g i n a t e d
in a c i r q u e d i r e c t l y sou th o f Sacagawea Peak.
However, t h e Cache
Creek i c e was squeezed and shunted by t h e much l a r g e r i c e complex from
F a i r y Creek on t h e n o r t h and t h e Ross Peak complex on t h e sou th.
66
T i l l from t h e Cache Creek i c e oc cur s in morainal form as well as a
fo r m le s s co ver o f ground moraine.
re c o rd e d .
Three s t a d e s o f g l a c i a t i o n a r e
Advance I (Bull Lake?) i s t r a c e a b l e th r oug hout much o f th e
n o r t h e r n h a l f o f Sec. 25, T 2 N, R 6 E.
I t has l i t t l e to po g ra p h ic
e x p r e s s i o n e x c e p t in t h e extreme NE %, Sec. 25, T 2 N, R 6 E where i t
has a subdued, low c r e s t e d , l o b a t e shape. . The remainder o f t h e moraine
a pp ear s in s h e e t form with l a r g e , smoothed blocks o f lim e s to n e and
s an ds to ne strewn over t h e s u r f a c e .
. Advances 2 and 3 ( P in e d a le I and I I ? ) a r e f u r t h e r u p v a ll e y from
Advance I and s e p a r a t e d from each o t h e r by approxim at el y I % m il e s .
Advance 2 i s in t h e NW k, Sec. 25, T 2 N, R 6 E, and Advance 3 i s
l o c a t e d in t h e NW
Sec. 26, T 2 N, R 6 E.
Advance 3 has a s t e e p e r
morainal f r o n t and b e t t e r developed k e t t l e s than Advance 2.
A l a r g e flow or l a n d s l i d e , a s s o c i a t e d with Advance 2, i s found
in t h e NW k» Sec, 25, T 2 N, R 6 E.
The s l i d e has shunted Cache Creek
i n t o a more n o r t h e r l y c our s e along i t s n o r th s i d e .
The B a t t l e Ridge F a u l t as well as a second, unnamed f a u l t west of
t h e B a t t l e Ridge F a u l t a r e t r a c e a b l e from t h e SE k, Sec. 30, T 2 N,
R 7 E i n t o t h e c e n t e r o f Sec. 31, T 2 N, R 7 E by lo n g , l i n e a r r i d g e s
o f bedrock, co ll u v iu m , and residuum.
The westernmost f a u l t from
B a t t l e Ridge i s a l s o marked by an i n t e r m i t t e n t st ream.
The Cross Range
F a u l t i s co ns p ic u o u s .b y t h e p resen ce o f seeps and s p r i n g s in t h e NE
Sec. 25, T 2 N, R 6 E.
67
W
E
Ross Psok
9000
Middle Branch o f North Fork
SOOO
7000
6000
North Fork of B rack ett
/
Creek
6000
Figure 17. Lon gi tu di nal P r o f i l e of Ross Peak Valley ( V e r t i c a l
e x a g g e r a ti o n i s 2.5X) (See P l a t e I f o r an e x p l a n a t i o n of th e symbols).
68
Upper Fork o f B r a c k e t t Creek
. Summary.
B r a c k e t t Creek begins a t t h e conflue nce o f i t s t h r e e
f o r k s . North, Middle, a nd .S o u th , on t h e s o u t h e a s t e r n boundary o f th e
stu dy a r e a .
The topography o f North Fork and South Fork has been
shaped by g l a c i a l a c t i v i t y .
Middle Fork o f B r a c k e t t Creek, in c o n t r a s t ,
shows t h e r e s u l t s t h a t s t r u c t u r e , s t r a t i g r a p h y , and p a s t changes of
c l i m a t e have on topography.
Three e a s i l y r e c o g n i z a b l e s t a d e s o f g l a c i a t i o n a r e d i s p l a y e d in
both North and South Forks.
o f residuum and col luv ium .
The n o n - i c e a r e a s a r e veneered by d e p o s i t s
I s o l a t e d p a tc h e s a r e a l l t h a t remain o f a
once more e x t e n s i v e outwash p l a i n .
S u r f i c i a l d e p o s i t s o f Middle Fork o f B r a c k e t t Creek a r e composed
predom ina ntl y o f c o llu viu m , residuum, and i s o l a t e d exposures o f bedrock.
A number o f l a n d s l i d e s a l s o e x i s t in t h i s d r a in a g e .
A small a re a in t h e
s o u t h e a s t e r n p a r t o f Middle Fork Basin has been i c e sc o u r ed .
The a r e a
c o n t a i n s a small p r o t o - c i r q u e and i t s a s s o c i a t e d t i l l d e p o s i t s .
North Fork o f B r a c k e t t Creek B a s i n .
The major i n f l u e n c e upon
topography in t h e North Fork d r a in a g e was a l a r g e i c e lobe o r i g ­
i n a t i n g in t h e c i r q u e beneath and n o r t h e a s t of Ross Peak.
The .
c i r q u e , in t h e SW %, Sec. 2, T I N, R 6 E, i s d e s i g n a t e d t h e Ross
Peak c i r q u e ( s e e f i g u r e 18).
Smal le r i c e lobes added t o t h e
i c e mass from Ross Peak and c o a le s c e d t o form t h e l a r g e te rm in a l
69
Fig ur e 18. Ross Peak c i r q u e and t h e U-shaped Ross Peak Valley
Foreground i s t h e complex of morainal m a t e r i a l o f Advance I (Bull Lake?)
o f North Fork.
70
moraine o f Advance I (Bull Lake?) found in th e n o r th e r n q u a r t e r of
Sec. I , I I N, R 6 E and t h e e a s t e r n . h a l f o f Sec. 36, T 2 N, R 6 E.
This moraine has a narrow, low, subdued c r e s t th rou ghout most o f i t s
a r c u a t e shape.
I t d o e s, however, have a broa d, l a r g e l y to pogr a phic
e x p r e s s io n with k e t t l e s . i n t h e SE %, Sec. 36, T 2 N, R 6 E.
A small
outwash remnant extend s from t h e moraine in t h e SW %, Sec. 31, T 2 N,
R 6 E.
Moraines o f Advance 2 ( P in e d a le I?) l i e d i r e c t l y behind and
u p v a l l e y from Advance I in t h e w e ste rn h a l f o f Sec. 36, T 2 N, R 6 E.
Small m e lt w a te r channelways s e p a r a t e t h e s e moraines from t h e o l d e r one
The chann el s breech t h e o l d e r moraine in two p l a c e s in t h e SE %,
Sec. 36, T 2 N, R 6 E.
The moraines a r e d i s t i n c t , s e p a r a t e e n t i t i e s
forming a complex o f t e r m i n a l , medial and l a t e r a l moraines which can
be t r a c e d upstream t o i n d i v i d u a l c i r q u e s .
These younger moraines have
f r e s h e r ap pe a rin g topography with many more k e t t l e s .
In f a c t , they
have t h e b e s t developed to p o g ra p h ic e x p r e s s i o n o f a l l t h e moraines in
the basin.
Because o f t h e l o c a t i o n of t h e younger moraines so c l o s e l y behind
t h e o l d e r mora ine , much o f t h e t i l l o f t h e o l d e r advance may have been
washed away in t h e m e lt w a te r b e f o r e i t was entre nch ed and breeched th e
o l d e r morain e.
T h i s , in p a r t , may e x p l a i n t h e subdued, topography of
t h e o l d e r moraine (s e e f i g u r e 19).
71
Figure 19. Su r fa ce e x p r e s s io n o f Advance I (Bull Lake?) o f th e North
Fork o f B r a c k e t t Creek in c e n t e r o f photo.
72
Figure 20.
R 6 E.
Toe of an a c t i v e l a n d s l i d e in t h e SW %, Sec. 25, T 2 N,
73
Advance 3 ( P in e d a le I I ? ) i s r e p r e s e n t e d by moraines in t h e e a s t e r n
h a l f o f Sec. 35, T 2 N, R 6 E and in t h e SE %, Sec. 2, T l N, R 6 E.
Both moraines a r e p o o rl y exposed.
The moraines have a d e f i n i t e l o b a t e
shape but few s u r f a c e boul de rs and k e t t l e s .
A small outwash remnant
e xte nds downstream from t h e moraine in t h e SE %, Sec. 2, T I N, R 6 E .
f o r about a q u a r t e r o f a m il e .
Three small l a n d s l i d e s in t h e SE
Sec. 35, T 2 N, R 6 E modify
t h e l a r g e l a t e r a l - m e d i a l moraine s e p a r a t i n g t h e Ross Peak v a l l e y from
t h e f i r s t o f t h e s m a l l e r g l a c i a l v a l l e y s n o r th o f Ross Peak.
l a n d s l i d e s a r e composed o f t i l l .
These
The to p o g ra p h ic e x p r e s s i o n o f t h e
t o e p o r t i o n o f t h e s e s l i d e s i s subdued and g e n e r a l l y d i f f i c u l t to
recognize.
L a n d s li d e c h a r a c t e r i s t i c s in c l u d e a bowl-shaped headward
s c a r and l o n g i t u d i n a l r i d g e s p a r a l l e l t o t h e s l i d e d i r e c t i o n .
Commonly, l a n d s l i d e s c a r s in t h e a r e a have some regrowth of
g r a s s e s and shrubs b u t r a r e l y t r e e s .
Also, old l a n d s l i d e s on t h e North
Fork and i t s branches a r e o f t e n a s s o c i a t e d with old f o r e s t c l e a r c u t s .
In t h e w e ste rn q u a r t e r o f Sec. 2, T I N, R 6 E along t h e base of
t h e g l a c i a l l y o v e rs te e pe ned wall o f Ross Peak Valley a r e a number of
mud fl ow( ?), e a r t h f l o w ( ? ) , l a n d s l i d e , a v a la n c h e , and d e b r i s avalanche
deposits.
These d e p o s i t s o r i g i n a t e in t h e Big Snowy and Amsden u n i t s
which compose t h e w a l l .
I s o l a t e d , small outc ro p s o f t h e u n i t s p r o j e c t
through t h e co nfu si on o f t h e s u r f i c i a l d e p o s i t s along t h e s lo p e .
74
S e c ti o n 3.1 and t h e we stern h a l f o f Sec. 32, J 2 N, R 7 E a r e
u n d e r l a i n com ple tely by t h e e q u i v a l e n t s o f t h e Cokedale and Miner
Creek Formations o f t h e L i vin gst on Group.
Here s u r f i c i a l d e p o s i t s
oc cur among ou tc ro ps of be drock, e x t e n s i v e a r e a s of c o l l u v i a l s l o p e s ,
and pockets and more le v el a r e a s o f residuum.
Middle Fork o f B r a c k e t t Creek B a s i n .
S u r f i c i a l d e p o s i t s of
Middle Fork a r e mostly residuum and colluvium.
This s e c t o r o f the
st udy a r e a c o n t a i n s a g r e a t number of bedrock exposures as most o f t h e
a r e a has not been d i r e c t l y a f f e c t e d by i c e .
The e x c e p ti o n s a r e two
small a r e a s n e a r t h e s o u t h e a s t e r n margin o f t h e b a s i n .
One a re a i s t h e
p r o t o - c i r q u e ( f o ll o w in g t h e use o f J a c o b s , 1969) p o r t i o n (s e e f i g u r e 21).
Here a small tongue o f i c e extended a s h o r t d i s t a n c e from t h e p r o t o ­
c i r q u e t o an e l e v a t i o n o f about 6800 f e e t .
Near t h i s e l e v a t i o n , t h e
i c e d e p o s i t e d small l a t e r a l moraines and a small te rm in a l moraine.
The
second e x c e p ti o n i s in t h e n o r t h e r n h a l f of Sec. 13, T I N, R 6 E.
The
m a t e r i a l he re i s b e l i e v e d t o be t i l l from t h e South Fork lobe t h a t had
become s a t u r a t e d with m e lt w a te r and "flowed" t o i t s p r e s e n t p o s i t i o n
(s e e S u r f i c i a l Geology s e c t i o n f o r an e x p l a n a t i o n ) .
In t h e SE %, Sec. 11, T I N, R 6 E i s an a rea dominated by a c t i v e
and i n a c t i v e a val an ch e c h u t e s .
The t r a c k s , f r e e o f t r e e s , extend from
n e a r t h e c r e s t o f t h e Range t o a f l a t r unout a re a in t h e SE %, Sec. 11,
75
Figure 21.
P r o t o - c i r q u e o f t h e Middle Fork o f B r a c k e t t Creek.
76
T l N, R 6 E.
Trees do cover some o f t h e o l d e r , lower d e p o s i t s in th e
area.
Two f a u l t s , t h e Ross Peak F a u l t and t h e Pass F a u l t , bound Ross
Pass on t h e n o r th and sou th r e s p e c t i v e l y .
in p a r t , f o r t h e e x i s t e n c e o f Ross Pass.
These f a u l t s probably account,
Bedrock o f t h e E l l i s Group
and t h e Morrison Formation u n d e r l i e s t h e Pass.
A l a r g e a r e a of
c o llu vi um , developed on t h e Morrison Formation, l i e s d i r e c t l y
east
downvalley from Ross P a s s.
N, R 6
A small l a n d s l i d e in Sec. 11, T I
and
E
a l s o e x i s t s in t h e Morrison Formation below Ross Peak.
The s o u t h e a s t t r e n d i n g t r a c e o f t h e Pass F a u l t i s shown by an a re a
o f s p r i n g s and c o n t i n u a l l y s a t u r a t e d ground in t h e SE %, Sec. 12, T I N,
R 6 E.
This a r e a shows small hummocks, a g e n e r a l l y convex upward
s u r f a c e , and boggy p a t c h e s .
These f e a t u r e s r e s u l t e d from very slow
downs!ope movement.
Two a r e a s o f mass wast ing occu r in t h e SE %, Sec, 2, T l
and in t h e SW %, Sec. ! , T l
N, R 6 E.
The f i r s t . In. Sec. 2,
N, R 6
E
i s an
a r e a o f b ro a dl y r o l l i n g t o hummocky topography not p o s s e s s i n g lo b a t e
form.
This m a t e r i a l resembles t i l l .
Indeed, i t may have been.
However,
su bsequent t o d e p o s i t i o n i t underwent downs!ope movement and came to
r e s t in i t s p r e s e n t p o s i t i o n and r e s u l t a n t form.
I t i s s i m i l a r in
appearance t o t h e f l o w t i l l mentioned in t h e n o r t h e r n h a l f o f Sec. 13,
T I N, R 6 E and i s a l s o c a l l e d f l o w t i l l .
77
Across t h e n o r t h Branch o f Middle Fork in t h e SW
R 6 E is a th ird , larger slide.
Sec. ! , T l
N,
This s l i d e i s near t h e c o n t a c t of the
Kootenai Formation and t h e Colorado Group.
I t i s m os tly in t h e Colorado.
The s l i d e extend s downvalley almost t o t h e North Branch o f Middle Fork.
I t begins ne ar t h e swale in t h e summit o f t h e r i d g e s e p a r a t i n g th e North
Branch o f Middle Fork o f B r a c k e t t Creek from th e South Branch of th e
North Fork o f B r a c k e t t Creek.
In i t s lower p o r t i o n s , t h e s l i d e i s
a c t i v e l y p r o g r e s s i n g as evidenced by t e n s i o n cra ck s and "drunken" t r e e s .
The upper p o r t i o n s have boggy a r e a s and s p r i n g s .
The most s t r i k i n g f e a t u r e s o f t h e lower reaches o f Middle Fork
i n c lu d e e x t e n s i v e , open, p a r k - l i k e a r e a s and a b ra id e d p a t t e r n of
Middle Fork n e a r t h e j u n c t i o n o f t h e t h r e e f o r k s o f B r a c k e t t Creek.
The open a r e a s a r e developed in t h e e a s i l y eroded mudstone o f th e
Hoppers and Billman Creek Formations o f t h e L iv in gst on Group.
The
br a id e d p a t t e r n o f Middle Fork i s t h e r e s u l t o f f a u l t c o n t r o l by th e
B a t t l e Ridge F a u l t t o t h e e a s t and an unnamed f a u l t t o t h e west
ex ten di ng sou th from B a t t l e Ridge (s e e f i g u r e 4 ) .
At t h i s l o c a t i o n .
Middle Fork i s flowing t r a n s v e r s e t o a graben formed by t h e down­
dropping o f t h e block between t h e f a u l t s . . The B a t t l e Ridge F a u lt
i n h i b i t s t h e f r e e flow o f Middle Fork and f u r t h e r i n f l u e n c e s th e
b ra id e d p a t t e r n seen on t h e map.
78
South Fork o f B r a c k e t t Creek B a s i n .
The South Fork o f B ra ck e tt
Creek, f o r t h e most p a r t , i s a narrow g l a c i a l v a l l e y ( s e e f i g u r e 22).
Evidence f o r a t l e a s t t h r e e advances o f i c e i s found in t h e c onfi gu­
r a t i o n o f t h r e e s e p a r a t e and d i s t i n c t te rm in a l moraines.
The o l d e s t g l a c i a t i o n . Advance I ( P in e d a le I ? ) , i s recorded by
f r e s h a pp ea ri ng t i l l
in t h e NE %, Sec. 18, T I N, R 6 E.
The moraine
in t h i s l o c a l i t y d i s p l a y s very marked knob and k e t t l e topography,
p o o rl y i n t e g r a t e d d r a i n a g e , and i s l i b e r a l l y strewn with bould ers of
lim e s to n e and sa n d s to n e .
60 f e e t .
The South Fork has notched t h i s moraine 40 t o
The termi nus has a l o b a t e shape which merges upv a ll e y on t h e
sou th with a w e l l - d e f i n e d l a t e r a l moraine.
East and downstream from
t h e m ora ine , outwash i s co nfin ed between bedrock r i d g e s o f t h e
Li vin g st o n Group.
The outwash i s t r a c e a b l e from t h e te rm in a l moraine
downstream t o t h e c o nfl ue nce o f t h e North Branch o f South Fork and t h e
South Fork.
In t h e w e ste rn h a l f of Sec. 18, T l N, R 6 E and upstream from
Advance I i s a second t e rm in a l morain e, Advance 2 ( P i n e d a l e I I ? ) .
K e t t l e s o f t h i s moraine a r e b r o a d e r , s l i g h t l y d e e p e r , and more widely
spaced than th o s e o f t h e o l d e r moraine.
a c c e n t u a t e d a r c u a t e form.
This moraine a l s o has a more
The l a t e r a l moraine of t h e n o r t h s i d e merges
almost i m p e r c e p t i b l y with t h e main s l o p e o f t h e v a l l e y a t about th e
s e c t i o n l i n e s e p a r a t i n g Sec. 13, T I N, R 6 E from Sec. 18, T I N,
79
Figure 22. View west toward summit o f B ri dge r Range showing narrow
U-shaped g l a c i a l v a l l e y o f t h e South Fork o f B r a c k e tt Creek from
Montana Highway 293 (middle c e n t e r o f p h o t o ) .
80
8000
7000
Qm
South Fork
South Fork
6000
Figun? 23. Long it udi nal P r o f i l e of South Fork of B r a c k e t t Creek
( V e r t i c a l e x a g g e r a t i o n is 2.5X) (See P l a t e I f o r an e x p l a n a t i o n
of t h e s ym bo ls ).
81
9000
,C re st of Range
Z3f
Middle Branch North Fork
8000
7000
7000
Figure 24. T r an s ve rs e P r o f i l e of Ross Peak Valley (Northern k, Sec. 2,
T I N, R 6 E) ( V e r t i c a l e x a g g e ra ti o n i s 2.5X) (See P l a t e I f o r an
e x p l a n a t i o n o f t h e s ym bol s) .
s
N
South Fork
7000
6000
Figure 25. T r an s ve rs e P r o f i l e o f South Fork of B r a c k e t t Creek (Center
Sec. 13, T I N, R 6 E) ( V e r t i c a l e x a g g e r a ti o n i s 2.5X) (See P l a t e I f o r
an e x p l a n a t i o n of t h e s ym bol s) .
'82.'
R 7 E.
No w e l l - d e f i n e d l a t e r a l moraine, e x i s t s from here t o t h e c e n t e r
h a l f of S e c t i o n 13.
t h e n , around t h e NW
The w e l l - d e f i n e d p o r t i o n o f t h e l a t e r a l resumes,
Sec. 13, T I N, R 6 E l i n e .
This segment of
t h e l a t e r a l moraine i s about 200 f e e t h i g h e r than i t s c o u n t e r p a r t below
and downstream.
No hard ev idence e x i s t s f o r mass wa sti ng o f t h i s
segment i n t o t h e South Fork.
The " f l o w t i l l " o f Middle Fork o f B r a c k e tt
Creek may be t h e remains o f t h e moraine.
Also , t h e r e i s a meltw at er
channel d r a i n i n g i n t o t h e North Branch o f South Fork which heads in t h e
missing p o r t i o n o f t h e l a t e r a l moraine.
This c o u l d , in f a c t , account
f o r a segment o f t h e m is si ng moraine.
A t h i r d moraine. Advance 3 ( P i n e d a le I I I ? ) , i s l o c a t e d near th e
c e n t e r o f Sec. 13, T I N,.R 6 E.
This moraine la ck s t h e l a r g e number .
o f k e t t l e s po s se ss e d by t h e o t h e r two advances b u t , n o n e t h e l e s s , has
some.
Su r fa ce b o u ld e rs a r e a l s o l e s s f r e q u e n t .
The moraine f r o n t
r i s e s q u i c k l y about 20 t o 30 f e e t above t h e v a l l e y f l o o r .
A f a i r l y r e c e n t l a n d s l i d e b u r i e s t h e south s i d e o f t h e terminus o f
Advance 3.
The s c a r i s r e a d i l y v i s i b l e .
e n t i r e l y o f t i l l of t h e second advance.
The s l i d e i s composed
D i r e c t l y eastw ard and a d j a c e n t
t o t h e l a n d s l i d e i s an a re a on t h e v a l l e y s i d e which a ppea rs t o be
almost c o n t i n u a l l y w a te r s a t u r a t e d .
I t a l s o appears t o be undergoing
very slow downsTope movement.
There a r e two s m a l l e r l a n d s l i d e s in t h e NW %, Sec. 13, T I N, R 6 E
near the cirque thre shold.
The e a s t e r n one heads in t h e Morrison
83
Formation, and t h e d e b r i s extends almost t o t h e v a l l e y f l o o r .
Deposits
o f t h i s l a n d s l i d e a r e b a r e l y v i s i b l e which a t t e s t s t o t h e age of th e
s l i d e , or t h e la c k o f competency o f t h e m a t e r i a l t o s ta n d in a d i s t i n c ­
t i v e morphological form or both.
fo r m a t i o n s .
The w e st e rn s l i d e heads in J u r a s s i c
I t s d e p o s i t s a r e more d e f i n i t i v e as th e y do s ta n d in a
very subdued but r e c o g n i z a b l e a r c u a t e form.
The South Fork c i r q u e , southernmost o f t h e major i c e sou rces in t h e
s tu d y a r e a , does not have t h e c l a s s i c bowl o r ampit h e a t e r shape l i k e
th o s e t o t h e n o r t h .
I n s t e a d , i t i s e l o n g a t e and s u b p a r a l l e l t o th e
c r e s t o f t h e Range (s e e f i g u r e 26).
so u th er n ends.
Cols bound i t s n o r t h e r n and
The c i r q u e o u t l i n e fo ll o w s somewhat t h e outc rop p a t t e r n
o f t h e Big Snowy Group-Amsden Formation.
These u n i t s a r e n o ta b ly non-
r e s i s t a n t and, t h e r e f o r e , prime l o c a l e s f o r easy removal by ic e and mass
w a s ti n g .
The a r e a d i r e c t l y sou th and s o u t h e a s t o f South Fork c o n ta i n s
exposures o f bedrock, residuum, and colluv ium.
A particularly intrigu­
ing a re a l i e s d i r e c t l y south o f t h e l a t e r a l moraine o f Advance 2 in t h e
s out her n q u a r t e r o f Sec. 13, T I N, R 6 E.
This a r e a c o n t a i n s t i l l
which i s much more subdued than t h a t o f South Fork.
Presumably, th e
t i l l o r i g i n a t e d in t h e now almost c om ple te ly t a l u s - f i l l e d c i r q u e
d i r e c t l y south o f t h e South Fork c i r q u e .
a remnant o f outwash.
A ss oc ia te d w it h t h e t i l l i s
The n o r th Branch o f Maynard Creek now roughly
approximates t h e l o c a t i o n o f t h e outwash.
.
84
Figure 26. J u x t a p o s i t i o n of South Fork c i r q u e and P r o t o - c i r q u e of
Middle Fork. Deposits in th e middle ground a r e " f l o w t i l l ."
85
CORRELATION OF GLACIAL STAGES OF THE STUDY AREA
C o r r e l a t i o n s o f t h e advances o f i c e a c r o s s t h e s tu d y a r e a a r e
based on comparisons with t h e sequence e s t a b l i s h e d a t F a i r y Creek.
These c o r r e l a t i o n s a r e on a r e l a t i v e - a g e r a t h e r than an a b s o l u t e - a g e
. basis.
The c r i t e r i a used have no d e f i n i t e numerical age d a t e s and,
t h e r e f o r e , can only e s t a b l i s h an o l d e r o r younger sequence o f s t a d e s .
The fo ll o w i n g a r e t h e c r i t e r i a upon which t h i s h y p o th e s i s i s based:
1)
The c i r q u e s a r e o f ap pro xim a te ly equal dimensions and
volumes and a r e c u t in t h e same ty pe of bedrock. This
s u g g e s t s synchronous i n c e p t i o n and development. S p e c i f i c
d i f f e r e n c e s can be e x p la i n e d by v a r i a t i o n s in e l e v a t i o n
and a r e a s o f catchment. Older c i r q u e s would have formed
very s h e l t e r e d a r e a s f o r r a p i d development o f younger
ones. The younger ones l i k e l y would have c u t both more
deep ly and f u r t h e r westward i n t o rocks o f o l d e r age than
t h e Madison Group.
2)
The moraines e x h i b i t a comparable degree o f to po g ra p h ic
development and form of s u r f a c e f e a t u r e s . These f e a t u r e s
in c l u d e h e i g h t and sh ar pne s s o f c r e s t s and pre s e n c e or
absence o f b o u l d e r s .
3)
The g l a c i a l d e p o s i t s a r e d i v i s i b l e i n t o a t l e a s t t h r e e
d i s c r e e t u n i t s in each o f t h e b a s i n s .
4)
The d e p o s i t s of each i n d i v i d u a l advance in t h e g l a c i a t e d
b a s i n s a r e s i m i l a r in e l e v a t i o n ; t h i s s u g g e s ts time e q ui va ­
l e n c e . The ex ce p ti o n i s Cache Creek where each advance
o c cu r re d a t h i g h e r e l e v a t i o n s th a n in t h e o t h e r d r a i n a g e s .
Each o f t h e s e c r i t e r i a ta ken s e p a r a t e l y could probably be ex p la i n ed by
o t h e r mechanisms.
However, c o n s id e re d t o g e t h e r , t h e s e f a c t o r s provide
s t r o n g ev ide nc e f o r b a s in t o ba sin c o r r e l a t i o n .
86
The g l a c i a l d e p o s i t s in t h e s tu dy a r e a a r e t e n t a t i v e l y c o r r e l a t e d
with t h e c l a s s i c a l Rocky Mountain sequence (s e e T a b l e - 4 ) .
Thus, th e
o l d e s t advance. Advance I , a t Fa ir y Creek, Cache Creek, and North
Fork o f B r a c k e t t Creek i s c o r r e l a t e d with t h e Bull Lake Stage o f the
c l a s s i c a l sequence.
Thi s c o r r e l a t i o n i s based upon:
I) s tra tig ra p h ic
p o s i t i o n ( t i l l o f t h i s age i s o v e r l a i n , a t l e a s t a t F a ir y Creek, by
younger t i l l ) ; 2) f u r t h e s t e x t e n t from t h e c i r q u e ; 3) subdued morpho­
l o g i c form; and 4) fewer s u r f a c e boul de rs than younger moraines.
Advance 2 a t F a ir y Creek, Cache Creek, and North Fork o f B r a c k e t t Creek
i s c o r r e l a t e d with t h e P in e d a le I o f t h e c l a s s i c sequence.
Advances 3
and 4 o f F a ir y Creek.and t h r e e o f t h e remaining d r a i n a g e s a r e c o r r e l a t e d
w it h t h e P in e d a le I I and Pi n e d a le I I I advances r e s p e c t i v e l y o f th e
c l a s s i c a l Rocky Mountain sequence.
The c o r r e l a t i o n s o f Advances 2, 3,
and 4 o f t h e stu dy a r e a with P in e d a le I , I I , and I I I a r e made on the
b a s i s o f:
I ) t h e p o s i t i o n o f t h e s u c c e s s i v e l y younger moraines f u r t h e r
upstream from ea c h, th us c l o s e r to t h e c i r q u e s ; 2) t h e f r e s h e r appearing
topography o f s u c c e s s i v e advances; 3) more s u r f a c e b o u l d e r s ; 4) poorly
i n t e g r a t e d d r a i n a g e ; and 5) r e s p e c t i v e l y s m a l l e r s i z e o f t h e moraines
wit h each advance.
C o r r e l a t i o n i s more d i f f i c u l t w i t h i n t h e South Fork o f B ra c k e tt
Creek b a s i n .
D ep os its o f Advance I in t h i s basi n e x h i b i t a l e s s
modified to p o g r a p h ic s u r f a c e than t h e i n i t i a l advances in t h e o t h e r
d r a i n a g e s and a l s o c o n t a i n a g r e a t e r number o f s u r f a c e b o u l d e r s .
It
87
does not appea r to be as old as Advance I o f t h e o t h e r d r a i n a g e s .
R a t h e r, i t more c l o s e l y resembles Advance 2 of t h e o t h e r d r a i n a g e s .
If
t h i s c o r r e l a t i o n i s c o r r e c t , then Advance I o f South Fork corresponds
to P in e d a le I o f t h e c l a s s i c a l sequence.
Advances 2 and 3 o f South
Fork, on t h e b a s i s o f f a r t h e s t e x t e n t and to po g ra p h ic e x p r e s s i o n ,
co rrespond t o Advances 3 and 4 o f F a i r y Creek and c o r r e l a t e with th e
Pi n e d a le I I and I I I o f t h e c l a s s i c a l sequence.
In o t h e r words, t h e
ba si n i s one s t a d e o u t o f phase with t h e o t h e r s .
This becomes confusing
because t h e ba si n meets t h r e e o f t h e f o u r c r i t e r i a used f o r basi n to
b a s in c o r r e l a t i o n .
I t may be t h a t an u b i q u i t o u s d e p o s i t in t h e SE %,
Sec. 18, T I N, R 7 E i s indeed t i l l which c o r r e l a t e s w it h Advance I
of the other drainages.
On t h e o t h e r hand, t h e South Fork basi n may
not have been s u b j e c t e d t o many g l a c i a t i o n s .
Therefore, the f a r th e s t
d e f i n i t e t i l l downstream r e p r e s e n t s t h e o l d e s t advance and i s a
P in e d a le I e q u i v a l e n t .
Table 4.
C o r r e l a t i o n o f G la c ia l Advances o f th e Study Area
Cache Creek
North Fork
South Fork
C l a s s i c a l Sequence
F a i r y Creek
P in e d a le I I I
Advance 4
Pi n e d a le II
Advance 3
Advance 3
Advance 3
Advance 2
P in e d a le I
Advance 2
Advance 2
Advance 2
Advance I
Bull Lake
Advance I
Advance I
Advance I
Advance 3
88
OTHER GLACIATED AREAS. OF THE BRIDGER RANGE'
Other a r e a s on t h e e a s t and west f l a n k s o f t h e B ri dge r Range appear
t o have undergone g l a c i a t i o n .
These were examined only b r i e f l y ; some
were examined on a i r photos and o t h e r s by a s h o r t f i e l d v i s i t .
These
may e v e n t u a l Iy be c o r r e l a t e d w ith t h e st ud y a re a i f f u r t h e r work i s
done by o t h e r s .
The a re a d i r e c t l y n o r th o f t h e p r e s e n t stu dy may be
used as an example.
present.
A number of c i r q u e s and U-shaped v a l l e y s a re
This segment o f t h e Range i s d r a in e d by F r a z i e r Creek, Carrol
Creek and t h e South Fork o f Fla th e a d Creek.
A p o r t i o n o f t h e Carrol
Creek d r a i n a g e i s in c lu d e d in t h e extreme nort h w e s te r n p a r t o f th e
p r e s e n t s tu dy a r e a .
The F r a z i e r Creek-South Fla th e a d s e c t i o n a l s o
in c l u d e s t h e Ha rd sc ra bbl e Peak c i r q u e and an ex tre me ly small p a r t of
t h e l a t e r a l moraine t h a t extends from i t (s e e P l a t e I ) .
The Hard­
s c r a b b l e Peak c i r q u e i s f i l l i n g with t a l u s as a r e t h e c i r q u e s t o th e
s ou th .
The f l o o r o f t h e c i r q u e i s occupied by mudflow d e p o s i t s and a
t a r n , F r a z i e r Lake.
cirque threshold.
by bedrock.
Ainger Lake i s downvalley and perched below th e
I t i s bounded on i t s we stern and n o rt h w e s te r n sho res
T i l l comprises t h e remaining s h o r e l i n e s and t h e ba sin
itself.
There i s s t i l l a n o t h e r a re a in t h e headwaters o f Maynard Creek and
south of t h e t h e s i s a r e a which shows evidenc e o f g l a c i a t i o n .
Again,
a small segment o f t h e a r e a i s inc lu de d in t h e p r e s e n t s tu dy a r e a (see
t h e d i s c u s s i o n of t h e South Fork B a s in ) .
This a rea c o n t a i n s t i l l
89
b e l i e v e d t o have o r i g i n a t e d in a c i r q u e in t h e NW
Sec. 24, T I N,
R 6 E (s e e f i g u r e 27).
i s p i l e d in t o
M at e ri a l t h a t resembles t i l l
subdued l o b a t e shapes and i s abundant in t h e B rid ge r Bowl Ski Area.
The c i r q u e s here a r e a l s o f i l l i n g with t a l u s .
Snow av al a nc hes a re
frequent also.
S m a l l e r , s t i l l l e s s well developed a m p h it h e a te r - s h a p e s a r e p r e s e n t
south o f t h e B ri d g e r Bowl Ski Area.
These may be t r u e c i r q u e s o r ,
p e rh a p s , p r o t o - c i r q u e s (s e e f i g u r e 28).
They a r e a l s o f i l l i n g with
t a l u s from t h e Madison Limestone.
There a r e a t l e a s t t h r e e major U-shaped canyons on t h e west s id e
of t h e Range (s e e f i g u r e 29).
wood Creek.
The f i r s t i s th e canyon o f North Cotton­
The v a l l e y p ro p e r c o n t a i n s numerous d e p o s i t s which appear
t o be l a t e r a l and t e rm in a l moraines (s e e f i g u r e 30).
The south s i d e
of t h e v a l l e y i s sh ould er ed which h i n t s a t deep gouging by m u l t i p l e
g l a c i a t i o n s (s e e f i g u r e 31).
The n o r th s i d e i s dominated by mass
w a st in g d e p o s i t s and rock g l a c i e r s ne ar t h e f l o o r .
The second canyon, f u r t h e r n o r t h , i s Mill Creek Canyon (s e e f i g u r e
29).
The Belgrade Quadrangle to p o g ra p h ic s h e e t shows Mill Creek
Canyon to be U-shaped.
In Sec. 5, T 2 N, R 6 E, two n o r t h f a c i n g ,
a d j a c e n t canyons e x h i b i t s c a l l o p e d topography s i m i l a r t o t h a t t y p i c a l
of ice a ffe c te d a re as.
These s u b s i d i a r y canyons a r e now d i s s e c t e d by
t h e i r re s p ec tiv e streams.
The middle segment o f Mill Creek Canyon
shows f a c e t e d s i d e s t o f u r t h e r s u g g e s t g l a c i a l a c t i v i t y .
90
Figure 27. Cirque o f North Branch of Maynard Creek (Note t h e f i l l i n g
of t h i s c i r q u e with t a l u s ) .
91
Figure 28. Typical e x p r e s s io n o f th e p r o t o - c i r q u e s (?) south of
th e stud y a r e a .
92
I
2
3
Figure 29. View eastward toward g l a c i a t e d v a l l e y s o f t h e west s i d e of
th e B ri d g e r Range:
1) Mill Creek Canyon
2) North Cottonwood Canyon
3) Tom Reese Creek
4
tV ^
93
Figure 30. View eastward up th e v a l l e y o f North Cottonwood Creek
(Note th e s hould ered v a l l e y s i d e ) . F l a t - t o p p e d peak in t h e background
i s Ha rds crab ble Peak.
94
31. Morainal choked f l o o r o f North Cottonwood Creek on west s i d e
o f B ri dg e r Range.
95
The t h i r d canyon d e f i n i t e l y shows t h e e f f e c t s o f g l a c i a l a c t i v i t y .
I t c o n t a i n s t h e middle and upper branches o f Tom Reese Creek (see
f i g u r e 32) .
Near Sacagawea Peak in Tom Reese Creek Canyon i s a U-shaped
v a l l e y t h a t e xten ds i n t o space well above t h e f l o o r o f t h e G a l l a t i n
Val ley .
Numerous p i l e s and lobes o f c o a r s e , a n g u l a r rock d e b r i s occupy
both s i d e s o f t h e v a l l e y (s e e f i g u r e 32).
These a r e very s i m i l a r in
appearance to t h e rock g l a c i e r s o f Madole (1972) and White (1975).
A s m a l l e r a r e a n e a r Tom Reese Creek and Sacagawea Peak a l s o h i n t s
at glacial a ctiv ity .
This i s t h e uppermost reach o f Corby Gulch.
It
has a U-shape and a l a r g e , ma ssi ve , a n g u l a r rock d e p o s i t ne ar i t s head
(s e e f i g u r e 33).
The ev ide nc e f o r e x t e n s i v e g l a c i a t i o n s on th e e a s t f l a n k o f th e
Range p l u s t h a t sug ges te d above f o r t h e west s i d e i n v i t e s s p e c u l a t i o n
about t h e g l a c i e r s o f t h e B ri dge r Range.
The f i r s t i s t h a t g l a c i a t i o n
c o n s i s t e d only o f l o c a l v a l l e y g l a c i e r s .
These were p r e s e n t on e i t h e r
s i d e o f t h e Range and flowed from s e p a r a t e , d i s t i n c t s ourc e a r e a s .
The
second i s t h a t l o c a l v a l l e y g l a c i e r fo rm ati on cu lminated in s m a l l, l o c a l
i c e caps.
D e g l a c i a t i o n allowed r e v e r s i o n t o v a l l e y g l a c i e r s which sub­
s e q u e n t l y e r a s e d a l l evidence f o r t h e i c e caps.
l a t i o n on t h e p a r t o f t h e w r i t e r .
This i s pure specu­
However, ic e caps have been p o s t u ­
l a t e d in t h e Madison Range and G a l l a t i n Range ( H a l l , 1 9 6 0 a ), in th e
Tobacco Root Mountains ( J a c o b s , 1969) and covering t h e Yellowstone
P l a te a u ( P i e r c e , 1976; Richmond, 1970).
96
Fig ur e 32. View westward down g l a c i a t e d upper v a l l e y o f Tom Reese
Creek (Note t h e rock g l a c i e r s on t h e south v a l l e y w a l l ) .
97
Figure 33. View southwestward down headwaters o f Corby Gulch.
l a r g e , l o b a t e form o f a n g u l a r boulde rs probably i s moraine.
The
98
QUATERNARY HISTORY
Many p re v io u s workers have atte m pte d t o e x p l a i n t h e Po s t-C re ta ce ous
geomorphic h i s t o r y o f so uthw estern Montana.
I t i s not t h e w r i t e r ' s
purpose to t r y any such r e c o n s t r u c t i o n f o r t h e B ri dge r Range.
R at her,
t h e goal i s t o c o n s t r u c t a p l a u s i b l e Qua terna ry h i s t o r y f o r th e e a s t e r n
f l a n k o f t h e Range based on d e t a i l e d s t u d i e s in t h e F a i r y Creek, Cache
Creek, and B r a c k e t t Creek a r e a s and re c o n n a is a n c e s t u d i e s o f o t h e r
p o r t i o n s o f t h e e a s t e r n B rid ge r Range.
For a more complete P o s t-
Cretaceous h i s t o r y , r e a d e r s a r e r e f e r r e d t o Alden (1932 and 1953),
McMannis (1955), Robinson (1963), Wolf (1964), and Roberts (1972).
McMannis (1.955) re co gnize d evidenc e f o r an old e r o s i o n s u r f a c e
s ur rou ndi ng t h e B ri d g e r Range.
The s u r f a c e i s p oorl y exposed and
t e r m i n a t e s a g a i n s t t h e c r e s t a l r i d g e o f t h e B rid ge rs a t e l e v a t i o n s o f
6400 f e e t a t t h e no rt h end of t h e Range, t o 7000 f e e t n e a r Fa ir y Lake,
and 6000 f e e t a t t h e sou th end o f t h e Range (McMannis, 1955).
He
c o r r e l a t e d t h i s s u r f a c e with t h e F l a x v i l l e s u r f a c e o f C o l l i e r and Thdm
(1918) and Bench No. I of Alden (1953).
McMannis (1955) b e li e v e d th e
age o f t h e s u r f a c e t o be m id - P li o c en e .
I t i s g e n e r a l l y b e l i e v e d t h a t sout hw est ern Montana a t t h i s time
c o n s i s t e d o f bro a d, n e a r l y l e v e l , d e b r i s - f i l l e d v a l l e y s with mountain
ranges o f low r e l i e f p r o t r u d i n g through t h e v a l l e y f i l l .
A Late
P l i o c e n e - E a r l y P l e i s t o c e n e r e a c t i v a t i o n o f re g i o n a l up a rc h in g , t i l t i n g .
99
and normal f a u l t i n g t h a t began in Miocene f u r t h e r u p l i f t e d t h e B ri dge r
Range and t h e old s u r f a c e .
D i s s e c t i o n o f t h e land masses began.
Thus,
t h e s t a g e i s s e t f o r t h e coming o f t h e P l e i s t o c e n e and g l a c i a t i o n .
No t i l l o r o t h e r d e p o s i t s o f d e f i n i t e p re -B ull Lake age have been
found on t h e e a s t f l a n k o f t h e B ri d g e r Range.
However, t h e r e a r e a
number o f f e a t u r e s in t h e stu dy a r e a t h a t , with f u r t h e r s t u d y , may be.
a s s i g n e d a p r e -B u ll Lake age.
These i n c l u d e :
I ) an extremely t h i n
cov er o f u b i q u i t o u s m a t e r i a l which o v e r l i e s L iv in gst on Group bedrock
from an o utc ro p in t h e SE
Sec. 18, T I N, R 7 E t o t h e c onflu en ce of
t h e North Branch o f South Fork and South Fork.
This m a t e r i a l i s d i s ­
continu ous with no a p p a r e n t form and i s t r a c e a b l e only by s c a t t e r e d ,
weathered bo uld e rs and cobb les o f s a n d s to n e .
bo u ld e rs en co u n te re d .
Very r a r e l y a r e lim e s to n e
2) T i l l in s h e e t - 1 ik e form with l a r g e li me st one
and a few sand st one bo uld e rs s c a t t e r e d throu ghout i t s e x t e n t i s found in
t h e SE h, Sec. 2, T I N, R 6 E.
I t i s in an anomalous p o s i t i o n with
rega rd t o t h e o t h e r advances in t h e North Fork b a s i n .
The p r e s e n t
ph ysi o g ra p h ic form r e s u l t e d l a r g e l y from t h e Bull Lake (?) g l a c i a t i o n
and was m odif ied by l a t e r advances,
in o r d e r t o d e p o s i t t i l l in t h i s
l o c a t i o n , i c e would have t o c r o s s t h e south Ross Peak v a l l e y wall in an
o b li q u e d i r e c t i o n t o t h e main t r e n d o f t h e v a l l e y and flow down t h e
South Branch o f North Fork o f B r a c k e t t Creek and t h e North Branch o f
Middle Fork o f B r a c k e t t Creek.
3) The c i r q u e a t Cache Creek i s
compound and i n c l u d e s a s m a l l , bro a de r c i r q u e i n t o which t h e main ci rque
TOO
has been i n c i s e d .
The s m a l l e r c i r q u e i s th oug ht t o be an o l d e r one,
alth ou gh i t could be a younger and not so well developed o n e - - a p r o t o ­
c i r q u e a c c ord in g to t h e use o f Jacobs (1969).
Deposits i n d i c a t i v e o f
a p r o t o - c i r q u e a r e la c k i n g which lends s u p p o rt to t h e ide a t h a t th e
s m a l l , broad c i r q u e i s indeed o l d e r .
bu ri e d o r o b l i t e r a t e d .
All o t h e r evidence i s e i t h e r
The g l a c i e r s o f t h i s age prob ab ly flowed from
small c i r q u e s down very shal lo w stream v a l l e y s .
have been stream v a l l e y s f o r t h e i c e t o e x p l o i t .
In f a c t , t h e r e may not
R a t h e r , t h e ic e simply
flowed from t h e c i r q u e s d i r e c t l y onto t h e F l a x v i l l e s u r f a c e .
If this
s u r f a c e was as p l a n a r as many workers g e n e r a l l y s u s p e c t , t h e g l a c i e r s
may no t have been a b l e t o b u i l d morain es.
I n s t e a d , such morainal
m a t e r i a l may simply have been s pre ad l a t e r a l l y by t h e m e lt w a te r in t h i n ,
g la cio -flu v ia l deposits.
Wolfe (1964) s t a t e s t h a t between pre-Wisconsin and Wisconsin and
even as l a t e as Wisconsin i n t e r g l a c i a l s , a d d i t i o n a l u p l i f t oc curred in
t h e Big B e l t and C a s t l e Mountains t o t h e n o r th and n o r t h e a s t of the
B ridgers.
These u p l i f t s , in combination w ith e r o s i o n , c u t canyons a t
l e a s t 200 t o 600 f e e t deep through t h e e x i s t i n g d e p o s i t s .
Richmond
(1965) a l s o s t a t e s t h a t 200 t o 1000 f e e t o f canyon c u t t i n g followed
t h e p r e -B u ll Lake g l a c i a t i o n s in o t h e r p a r t s o f t h e Rocky Mountains.
I f a comparable amount o f canyon c u t t i n g and e r o s io n d id occur in th e
B r i d g e r s , i t may have removed a l l t r a c e s o f g l a c i a t i o n on t h e F l a x v i l l e
surface.
This u p l i f t and some o f t h e canyon c u t t i n g may be r e f l e c t e d
101
in t h e B rid ge r Range by t h e s hould ered sou th v a l l e y wall o f North
Cottonwood Canyon in Secs. 17 and 18, T 2 N, R 6 E.
Also, t h e U-shaped
p o r t i o n o f t h e canyon o f Tom Reese Creek extends i n t o space well above
the f lo o r of the G a llatin Valley.
Bull Lake d e p o s i t s a r e r e p r e s e n t e d on t h e e a s t f l a n k s o f th e
B rid ge rs by t h e oute rmo st moraine a t F a i r y Creek with i t s outwash and,
p o s s i b l y , by Advance I o f Cache Creek and North Fork o f B r a c k e t t Creek.
E v i d e n t l y , p o s t Pre- Bul l Lake e r o s i o n had carved a l a r g e enough canyon
in t h e F a i r y Creek basi n to assume e s t a b l i s h m e n t of t h e p r e e x i s t i n g
drainage.
form.
Bull Lake i c e con tin ue d t o shape t h e ba si n i n t o t h e p r e s e n t
Other p a r t s o f t h e Range may not have had such a h i s t o r y .
This
i s su gge st ed by t h e problems in c o r r e l a t i o n , mentioned p r e v i o u s l y , and
by t h e even more subdued, s h e e t - l i k e n a t u r e o f t h e oute rm os t moraines
a t Cache Creek and North Fork o f B r a c k e t t Creek.
With t h e waning of
t h e g l a c i a l c l i m a t e , l a n d s l i d e s , slumps, and mudflows were probably
q u i t e p r e v a l e n t and e a s i l y developed on t h e s h a l y bedrock during times
o f abundant w a te r made a v a i l a b l e through m e lt in g o f t h e i c e .
The P i n e d a l e I advance a t F a i r y Creek most l i k e l y had a h i s t o r y
very s i m i l a r t o t h a t o f t h e Bull Lake.
Deposits of t h e Pin e da le I
advance a r e n e a r l y as l a r g e and extend almost as f a r as th o s e of Bull
Lake in a l l o f t h e b a s i n s .
E v i d e n t l y , t h e c l i m a t e had a m e l i o r a t e d enough by t h e beginning o f
P in e d a le II time t o s e r i o u s l y l i m i t t h e volume o f i c e produced.
This
102
l i m i t e d t h e e x t e n t o f t h e advances and t h e i c e was f u r t h e r confined
t o t h e v a l l e y s excav ated by t h e pre vio us and l a r g e r Bull Lake and
P in e d a le I advances.
P i n e d a le I I I i c e was r e s t r i c t e d t o t h e c i r q u e s a t F a i r y Lake and
Ross Peak and s l i g h t l y beyond them in South Fork which f u r t h e r a t t e s t s
t o th e changing c l i m a t e .
In o t h e r c i r q u e s , t a l u s accumulation and rock
g l a c i e r s began t o form.
Continued c l i m a t i c . a m e l i o r a t i o n caused t h e d i s a p p e a r a n c e of i c e
in t h e F a i r y Lake and Ross Peak c i r q u e s .
g l a c i e r growth fol lo we d.
Talus accumulation and rock
L a n d s l i d i n g , t h e d is ap p e a ra n ce o f rock
g l a c i e r s , and t a l u s accumulation r e s u l t e d in t h e o t h e r c i r q u e s .
Slump­
ing and l a n d s l i d i n g were p r e v a l e n t in t h e v a l l e y s a t t h i s time a l s o .
P o s t - g l a c i a l streams began t o i n c i s e c ours es i n t o t h e v a l l e y s as
l a n d s l i d i n g and g en eral to p o g r a p h ic smooting by t h e v a r i o u s weathering
p r o c e s s e s c ont in u e d .
This p o s t - g l a c i a l e r o s i o n i s b e l i e v e d t o be
r e s p o n s i b l e f o r t h e removal of s e v e r a l hundred f e e t o f m a t e r i a l from
t h e canyons o f t h e e a s t f l a n k o f t h e B rid ge r Range (McMannis, 1955).
LAND USE CONSIDERATIONS
In 1971 B ri d g e r Canyon came under a l e g a l zoning and planning code
Already p r e s e n t in t h e a r e a was a major r e c r e a t i o n s i t e , Bridger Ski
Area.
Due t o t h e amount o f a t t e n t i o n t h i s a re a has seen and t h e l i k e l y
i n c r e a s e in f u t u r e u s e , i t i s p e r t i n e n t t o d i s c u s s some o f t h e land use
103
a s p e c t s f o r t h e stu dy a r e a .
T h e r e f o r e , t h e fo ll ow in g d i s c u s s i o n o f
"environmental g e o lo g ic c o n d i t i o n s " and recommendations a r e given only
as f i r s t a pp ro xi m a tio ns .
s tu dy s t i l l
A f u r t h e r , more d e t a i l e d , comprehensive
remains t o be unde rta ken .
The d i s c u s s i o n here could not
a tt e m p t t o answer a l l t h e environmental q u e s t i o n s because th os e
q u e s t i o n s have not y e t been asked.
I t i s hoped, however, t h a t t h i s
s e c t i o n may a n t i c i p a t e some o f t h o s e q u e s t i o n s .
The approach o f t h e w r i t e r t o t h e environmental a s p e c t s i s through
a very ge ner al land c l a s s i f i c a t i o n system.
A th re e fo ld division of the
s tu d y a r e a was p o i n t e d out t o t h e w r i t e r (D. Smith, pe rs ona l communi­
c a t i o n , 1976) which may or may no t be a p p l i c a b l e t o t h e e n t i r e e a s t
f l a n k s o f t h e Range.
The d i v i s i o n s c o n s i s t o f:
I) the r e l a t i v e l y thin
s l i c e o f Car bonif erous and J u r a s s i c bedrock along th e west ern margin;
2) t h e c e n t r a l p o r t i o n covered by t i l l , and 3) th e e a s t e r n p o r t i o n
c o n s i s t i n g of bedrock o f t h e L iv in gst on Group veneered by residuum,
col luv ium , and al luv ium .
The geomorphological a s p e c t s o f each d i v i s i o n
have been documented in th e main t e x t o f t h i s pap er.
Therefore, the
environmental c o n s i d e r a t i o n s f o r each d i v i s i o n w i l l be made r a t h e r than
f o r t h e a r e a as a whole.
The a t t r a c t i v e n e s s o f t h e we stern p o r t i o n l i e s in j u s t those
f e a t u r e s t h a t d is c o u r a g e i t s use f o r much o t h e r than h i k i n g , l i m i t e d
camping, c r o s s c ount ry and a l p i n e s k i i n g , and i t s s c e n i c v a lu e.
The
a r e a i s c h a r a c t e r i z e d by very s t e e p s l o p e s , some high pe a k s , and the
n e a r l y kn if e -e d g e d c r e s t o f t h e Range.
The h e i g h t and s te e p n e s s of
t h e s lo p e s a r e conducive t o snow avalanc hes in w i n t e r and p a r t i c u l a r l y
in s p r i n g .
La nd s lid e s and flows occur in t h e n o t o r i o u s l y u n s t a b l e Big
Snbwy-Amsden u n i t s .
o f use.
These c o n s t i t u t e s e r i o u s l i m i t a t i o n s t o any form
The numerous t a l u s p i l e s and s l o p e s c h a r a c t e r i s t i c o f t h i s
d i v i s i o n a l s o put s e r i o u s c o n s t r a i n t s on f u t u r e use.
Much o f t h e t a l u s
i s being a c t i v e l y fed from both ch ute s and r o c k f a l l from t h e overhanging
cliffs.
Human h a b i t a t i o n i s hampered by t h e g r e a t amount o f snowfall in
t h e w i n t e r months, and t h e average te m p e ra t u re s a re very low.
te na nc e o f open roads and power l i n e s could be very c o s t l y .
Main­
Obtaining
ad equate w a te r and allow in g f o r dis posa l, o f w a s t e s . a l s o p r e s e n t s e r i o u s
drawbacks t o use because o f t h e impermeable n a t u r e o f t h e ge olo gic
m aterial.
This i s due t o t h e abundance o f c la y s and c l a y - s i z e d
m a t e r i a l from t h e s h a l e s in t h e Big-Snowy-Amsden u n i t s as well as some
of the J u ra ss ic u n its .
At p r e s e n t , t h e a r e a i s used p r i m a r i l y f o r i t s s c e n i c v a l u e ,
s k i i n g , and h i k i n g .
O c c a s i o n a l l y , domestic sheep a r e g r a z ed .
This
a r e a does p ro v id e h a b i t a t f o r a number o f d e e r , e l k , moose, and some
mountain g o a t s .
The c e n t r a l d i v i s i o n i s an a r e a o f g l a c i a l d e p o s i t i o n .
deposits are chiefly t i l l .
The
The a r e a has s t e e p s lo p e s in i t s western
p o r t i o n n e a r t h e boundary with t h e we ste rn d i v i s i o n as well as in t h e
105
e a s t e r n p a r t ne ar t h e boundary w ith t h a t d i v i s i o n .
The i n t e r v e n i n g
■
a r e a has both s t e e p s lo p e s and l e v e l a r e a s r e s u l t i n g from t h e c o n s t r u c ­
t i o n a l f e a t u r e s made by t h e g l a c i e r s .
a r e a l l in t h i s d i v i s i o n .
The l a n d s l i d e s o f t h e study area
The l a n d s l i d i n g probably develops from a
combination o f s t e e p s l o p e s and r e l a t i v e l y i n c o h e r e n t n a t u r e o f th e
t i l l when s a t u r a t e d .
This p a r t i c u l a r a r e a probably o f f e r s t h e b e s t
s i t e s f o r housing development.
o r on t h e v a l l e y s i d e s .
Development should n o t , however, be in
M o d if ic a t io n o f t h e e x i s t i n g s l o p e e q u i l i b r i u m
could e a s i l y r e s u l t in l a n d s l i d i n g .
Waste d i s p o s a l might p r e s e n t some d i f f i c u l t i e s .
The t i l l o r i g i ­
nated from s t r a t a high in c l a y c o n t e n t which may i n h i b i t r a p i d permea­
b ility.
F u r t h e r t e s t i n g f o r d r a in a g e c h a r a c t e r i s t i c s should be
conducted.
L i t t l e i s known o f groundwater a v a i l a b i l i t y in t h e a r e a .
The
numerous th r o u g h - f l o w i n g , p e r e n n i a l stream s su g g es t a r e s e r v o i r of
c o n s i d e r a b l e volume.
Water may be s t o r e d in t h e t i l l o r in th e Mesozoic
and Pa le o zo i c u n i t s a t de pth .
Water q u a l i t y should be checked f o r
suitability.
Major f a u l t s extend from t h e w e st e rn s e c t i o n i n t o t h i s a r e a .
These f a u l t s a r e marked by l i n e a r zones o f e x t e n s i v e s p r i n g s and by
a p p a r e n t slow downs!ope movement.
T h e r e f o r e , the y should be avoided.
The e a s t e r n d i v i s i o n o f a complex o f s t e e p , bedroc k-cor ed r i d g e s
106
wit h s l o p e s veneered by colluvium and open, le v e l a r e a s covered by
residuum.
Alluvium and outwash a r e found along t h e s tr e am s .
Any development in which e x c a v a ti o n o f m a t e r i a l i s involved should
s e r i o u s l y c o n s i d e r t h e bedrock l i t h o l o g i e s invo lve d.
t h e L iv in gs to n Group.
The bedrock i s
I t c o n ta i n s many beds o f b e n t o n i t e which, by i t s
g r e a t s h r i n k - s w e l l c a p a c i t y , s h a r p l y l i m i t s c o n s t r u c t i o n o f ro a d s ,
b u i l d i n g s , and t h e d r a in a g e of w a t e r .
The l a r g e r , open a r e a s a r e un de r­
l a i n by mudstone which may be very impermeable.
P r e s e n t l y , t h e s lo p e s o f t h e a re a a r e f a i r l y dry th r oug hout most
o f the year.
This may account f o r t h e i r s t e e p n e s s .
I f c l i m a t e became
w e t t e r , t h e b e n t o n i t e could c a t a l y z e l a n d s l i d e s o f s e r i o u s consequence.
In c o n c l u s i o n , i f any major c o n s t r u c t i o n in th e s tu d y a re a i s
p la nn ed , some very c a r e f u l thoug ht should be given t o t h e above g e o lo g ic
c o n s i d e r a t i o n s and many t e s t s conducted.
An e n g in e e r in g s u i t a b i l i t y
s tu dy o f a l l m a t e r i a l s , in c l u d i n g o n - s i t e i n v e s t i g a t i o n s , i s high ly
recommended.
A dd iti on a l s tu dy of t h e impact o f development on w i l d l i f e ,
and h a b i t a t s and seasonal d i s t r i b u t i o n and m ig r a ti o n o f t h e w i l d l i f e
would be b e n e f i c i a l .
Water q u a l i t y s t u d i e s could c o n t r i b u t e much
needed in fo r m a t io n on t h e hyd ro lo gic system and amount and s u i t a b i l i t y
o f t h e w a te r f o r use.
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_____ , 1962b, Qua terna ry s t r a t i g r a p h y o f t h e La Sal Mountains, Utah:
U. S. Geological Survey Pr of. Paper 324.
_____ , 1964a, Three pre -B u ll Lake t i l l s in t h e Wind River Mountains,
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p. 104-109.
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Montana, p. 112-113.
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Bi ennial Meeting, Miami, F l o r i d a , p. 47-49.
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t e c t o n i c h i s t o r y o f t h e Li vi n g st o n a r e a , sout hw est ern Montana: U. S.
Geological Survey P r o f . Paper 526-C.
113
_____ , 1963, The L i v in g st o n Group o f s o u t h - c e n t r a l Montana:
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Open F i l e Map.
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Montana S t a t e U n i v e r s i t y .
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A b s t r a c t s , v. 4, no. 6, Rocky Mountain S e c t. 25 Annual Meeting,
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c e n t r a l w e ste rn Montana: Geological S o c i e t y o f America B u l l . , v. 75,
p. 493-502.
APPENDIX
115
DETAILED STRATIGRAPHY
The fo ll o w i n g d i s c u s s i o n o f t h e s t r a t i g r a p h y o f t h e Bridg er Range
i s ta ken from McMannis (1955), Skipp and McGrew (1972), and Hackett and
o t h e r s (1960).
Precambrian
Bedrock ranging in age from Archean (?) t o Recent i s exposed in t h e
B r id g e r Range with t h e e xce pt io n of s t r a t a o f O r do vic ia n, S i l u r i a n ,
Permian and T r i a s s i c Ages.
The c r e s t o f t h e Range in most p l a c e s i s
rock o f M i s s i s s i p p i a n Age.
Rocks t o t h e west a r e o l d e r , and rocks to
t h e e a s t o f t h e c r e s t a r e younger.
Rocks o f Precambrian Age in t h e B ri d g e r Range c o n s i s t of c l a s s i c
rocks o f t h e B e l t Supergroup and a metamorphic basement complex.
The
metamorphic rocks in t h e so u th er n p a r t o f t h e we ste rn h a l f o f t h e
B rid ge r Range a r e pre dominantly b l a c k , w hit e and p in k , q u a r t z - f e l d s p a r
gneisses.
Some g a r n e t amphibole s c h i s t s a r e common showing g r a d a t i o n s
from g a r n e t a m p h ib o li te s t o g a r n e t - q u a r t z - f e l d s p a r g n e i s s .
Also
p r e s e n t a r e beds o f d o l o m i t i c marble.
In a d d i t i o n , small pe gmatites of
q u a r t z or w hit e o r t h o c l a s e a r e found.
These rocks a r e P r e - B e l t in
ag e, and t h e s c h i s t s , g n e i s s e s , and ma rbles a r e b e l i e v e d t o be meta­
sediments (McMannis, 1955).
The B e l t Supergroup r o c k s , in c o n t r a s t , a r e found only in th e
n o r t h e r n p a r t o f t h e w e ste rn h a l f o f t h e B r i d g e r s .
These rocks
116
a g g r e g a te 7-10,000 f e e t in t h i c k n e s s and a r e composed o f extremely
c o a r s e conglomerates t o a r k o s i c s an ds ton es in t h e s o u th e r n p a r t .
In
t h e n o r t h e r n p a r t , dark gray a r g i l l i t e s and s i l i c e o u s lim e st ones occu r.
The rock has a dark brownish-green weathered appearance and i s re d d is h
in o t h e r p l a c e s .
Fresh rock i s always mo derately dark g ra y -g re e n .
These s t r a t a a r e t h e e q u i v a l e n t s o f t h e LaHood Formation (McMannis,
1963) and p a r t s o f t h e Spokane and Greyson S ha le s.
Pa le o zo i c
Cambrian
S i x , well exposed fo r m at i on s of Cambrian Age a r e p r e s e n t west o f
t h e c r e s t o f t h e B ri d g e r Range.
These a r e , in ascending o r d e r of a ge,
t h e F l a t h e a d , Wolsey, Meagher, Park, P i l g r i m , and Snowy Range Formations..
The lowermost P a le o z o i c u n i t i s t h e Middle Cambrian Fl at he ad
Formation.
I t c o n s i s t s o f g r e e n i s h and r u s t y - r e d , y e l l o w , and white
f e l d s p a t h i c san ds to ne and a r k o s e .
In t h e a re a a d j a c e n t t o and d i r e c t l y
west o f t h e s tu dy a r e a , t h e Fl a th e a d i s 142 f e e t t h i c k (McMannis, 1955).
The n e x t younger fo rm ati on i s t h e Middle Cambrian Wolsey Shale.
Again, exposed d i r e c t l y west o f t h e s tu d y a r e a , t h e Wolsey i s a p p r o x i ­
mately 152 f e e t t h i c k .
I t r e s t s conformably on t h e F la th e a d Formation.
The Wolsey i s a g ra y -g re e n t o maroon, f i s s l e , micaceous s h a l e .
I t is
in te r b e d d e d with t h i n , r u s ty -b r o w n , la m i n a t e d , f i n e - g r a i n e d g l a u c o n i t i c
and micaceous s an ds to ne (McMannis, 1955).
117
Above t h e Wolsey and with a p p a r e n t co nformity i s t h e Meagher
Formation.
This for matio n i s a led ge former and has a c o n s i s t e n t
t h i c k n e s s o f ab out 370 f e e t .
I t can be div id e d i n t o t h r e e u n i t s :
I.) a lower u n i t o f l i g h t t o dark g r a y , t h i n bedded, f i n e - g r a i n e d
lim e s to n e w it h in te r b e d d e d f o s s i l i f e r o u s g r e e n i s h and y e l l o w i s h , s i l t y ,
c a l c a r e o u s s h a l e ; 2) a middle u n i t o f led ge or c l i f f forming dark g r a y ,
m a s s iv e , f i n e - g r a i n e d , dense l i m e s t o n e ; and 3) an upper u n i t o f l i g h t
t o dark g r a y , t h i n bedded, n o n - r e s i s t a n t , f i n e - g r a i n e d , f o s s i l i f e r o u s ,
dense lim e s to n e w it h s h a l e p a r t i n g s and beds (McMannis, 1955).
The Park Formation, o v e r l y i n g t h e Meagher, i s marked by a topo­
g r a p h i c low thr ou gh out t h e e x t e n t o f i t s o u tc r o p .
I t has a c o n s i s t e n t
t h i c k n e s s o f 188 f e e t and i s composed o f g r a y , brown, green and maroon,
f i s s i l e , micaceous s h a l e with some i n t e r c a l a t e d s an ds to ne beds
McMannis, 1955).
The P i l g r i m Formation i s ab out 433 f e e t t h i c k a t Sacagawea Peak
(McMannis, 1955).
I t i s made up o f two major u n i t s :
I ) a lower, g r a y -
brown, m a ss iv e , o o l i t i c , g l a u c o n i t i c , m o t t l e d lim e s to n e and a gray and
yellow-brown, t h i n t o medium bedded l i m e s t o n e , l im e s to n e peb ble con­
g l o m e r a t e , and edgewise conglomerate with in te rb e d d e d g r a y - g r e e n , f i s s i l e
and c a l c a r e o u s s h a l e ; and 2) an upper u n i t c o n s i s t i n g o f a medium-grained,
o o l i t i c , m o tt le d lim e s to n e (McMannis, 1955).
The m o t t l i n g i s b e li e v e d
t o o r i g i n a t e from p a r t i a l d o l o m i t i z a t i o n and r e c r y s t a l l i z a t i o n .
118
The Snowy Range Formation o f Late Cambrian Age conformably o v e r l i e s
t h e P i l g r i m Formation.
This for mation c o n s i s t s o f two members:
I) the
Dry Creek Shale member below; and 2) t h e Sage Pebble Conglomerate
member above (McMannis, 1955).
The Dry Creek Shale member i s approxim at el y 50 f e e t in th i c k n e s s
and c o n s i s t s o f g ra y -g re e n f i s s i l e s h a l e with in te rb e d d e d p a l e orange
t o yellow-brown, c a l c a r e o u s san dst one and s i l t s t o n e .
This member i s
ex pre sse d t o p o g r a p h i c a l l y as a swale above t h e P i l g r i m Limestone.
The Sage Pebble Conglomerate member i s 121 f e e t in t h i c k n e s s and
c o n s i s t s of b i o s t r o m a l , columnar l im e s to n e o v e r l a i n by p a l e yel low brown and p a l e o l i v e , t h i n to medium bedded, f i n e t o c o a r s e - g r a i n e d ,
g l a u c o n i t i c , f o s s i l i f e r o u s , lim e s to ne and lim e s to ne peb ble conglomerate.
The Maywood Formation spans t h e time boundary between Cambrian and
Devonian.
This for matio n c o n t a i n s a l l t h o s e rocks above d e f i n i t e
Cambrian rocks and below d e f i n i t e Devonian rocks (McMannis, 1955).
The
Maywood c o n s i s t s o f 92 f e e t o f red-brown s h a l e , r e d d i s h - o r a n g e , c a l ­
careous s i l t s t o n e , p a l e brow nis h-r ed l i m e s t o n e , p a le y e l l o w - g r a y s i l t st o n e and mudstone, gray-brown, f i n e g r a i n e d l i m e s t o n e , p a l e orangebrown, m a ss iv e , f i n e - g r a i n e d l im e s to n e .
O rdovician-Silurian
S t r a t a o f Ordovician and S i l u r i a n Age a r e a b s e n t in t h e Bridger
Range as th e y a r e in much o f w e ste rn Montana (McMannis, 1955) (McMannis
and Chadwick, 1964).
119
Devonian
The Devonian s t r a t a o f t h e B ri d g e r Range c o n s i s t o f t h e J e f f e r s o n ,
Three For ks, and Sappington f o r m a t i o n s .
The J e f f e r s o n Formation conformably o v e r l i e s t h e Maywood and i s
620 f e e t t h i c k j u s t n o r th of Sacagawea Peak (McMannis, 1955).
I t is
composed o f l i g h t and dark brown, medium t o t h i c k bedded dolom ite ,
d o l o m i t i c l i m e s t o n e , and li m e s to n e .
Interbed de d w it h t h e lim e st one s and
do lo mi te s a r e p a l e ye llo w and l i g h t pink d o l o m i t i c s i l t s t o n e s .
The
c a r b o n a te s weath er t o a rough ap pea rin g massive s u r f a c e w ith a d i s t i n c t
p e t r o l i f e r o u s odor and s a c c h a r d i d a l t e x t u r e .
Both Amphipora- sp. and
Stromatopora a r e l o c a l l y abundant in t h e f or m at io n.
Conformable with t h e J e f f e r s o n and d i r e c t l y above i t i s t h e Three
Forks Formation.
I t i s made up o f 155 f e e t o f g r e e n i s h - y e l l o w t o r e d ,
hackly s h a l e and g r e e n i s h - y e l l o w t o ora nge , ma ssi ve , c a l c a r e o u s s i l t s to n e b r e c c i a s ; a y e l l o w , brown, medium to t h i c k bedded, massive l e d g e ­
forming l i m e s t o n e , plu s a p a l e yellow-brown t o g r a y , t h i n bedded,
c a l c a r e o u s s i l t s t o n e o r s i l t y lim e s to n e with green s h a l e and a
p e r s i s t e n t gray lim e s to ne a t t h e top (McMannis, 1955).
A d i s c o n f o r m i t y s e p a r a t e s t h e Three Forks Formation from th e ov e rly in g Sappington Formation.
The 76 f o o t t h i c k Sappington Formation in
t h e B rid ge r Range c o n s i s t s o f bla ck f i s s i l e s h a l e ; p a l e brown to
yellow-brown, t h i n t o medium bedded, f i n e - g r a i n e d , c a l c a r e o u s s i l t s t o n e
120
and s a n d s to n e ; and dark brown to b l a c k , s i l t y s h a l e o r s i l t s t o n e
(McMannis, 1955).
M ississippian
The Madison Group, composed o f Lodgepole and Mission Canyon
For mat ion s, r e p r e s e n t s M i s s i s s i p p i a n rocks in t h e B r i d g e r s .
For t h e
most p a r t , t h e s e fo r m at i ons comprise t h e c r e s t o f t h e B ri d g e r Range.
Throughout much o f west ern Montana, t h e Mission Canyon forms c h a r a c ­
t e r i s t i c huge c a s t e l l a t e d r i d g e s , l e d g e s , and c l i f f s .
The Lodgepole Formation c o n s i s t s o f dark gray-brown, t h i n bedded
and p l a t y , f i n e - g r a i n e d lim e s to n e and i n t e rb e d d e d ye llo w w eathering
s h a l y l i m e s t o n e ; p a l e yellow-brown, orange-brown, gray-brown, and re ds t a i n e d , t h i n bedded, f i n e - g r a i n e d lim e s to n e in te rb e d d e d with medium
t o t h i c k bedded, s a c c h a r o i d a l l i m e s t o n e .
f o r m a ti o n .
F o s s i l s a r e abundant in t h i s
In f a c t , i t i s t h e most c o n s i s t e n t l y and abu ndantly f o s s i l -
i f e r o u s u n i t in t h e Rocky Mountain r e g i o n .
The Mission Canyon ranges in t h i c k n e s s a t Sacagawea Peak from 870
t o 950 f e e t (McMannis, 1955).
I t c o n s i s t s of p a l e yellow-brown, p oorl y
bedded li m e s to n e s w e at he rin g t o a l i g h t gray.
nodules a r e common.
Chert and limestone
C olla pse b r e c c i a s a r e a l s o common in t h i s formation.
M ississippian-Pennsylvanian
The Big Snowy Group and t h e Amsden Formation a r e rock u n i t s
121
r e p r e s e n t i n g t h e time i n t e r v a l t r a n s g r e s s i n g th e M i s s i s s i p p i a n Pennsylvanian boundary.
Big Snowy s t r a t a a r e not r e a d i l y a p p a r e n t in much o f t h e Bridger
Range (McMannis, 1955).
units:
However, t h e s e s t r a t a a r e r e p r e s e n t e d by t h r e e
I ) a red and pink d o l o m i t i c s i l t s t o n e and s h a l e w ith some purple,
m o t t l e d dolom ite and a few c a l c a r e o u s s an ds to ne beds; 2) a sandy u n i t o f
r e d , pink and p a l e y e ll ow -o ra nge s a n d s t o n e , s i l t y s a n d s t o n e , c a I c a r e n i t e , sandy dolom ite and red s i l t s t o n e ; and 3) a f o s s i l i f e r o u s dark
gr ay t o b la c k s h a l e , s h a l y l i m e s t o n e , and c h e r t y l im e s to n e (McMannis,
1955).
The Amsden Formation discon fo rma bly o v e r l i e s s t r a t a r e p r e s e n t i n g
t h e Big Snowy Group.
I t c o n s i s t s o f r e d , d o l o m i t i c s i l t s t o n e s and some
p u r p l e s h a l e s with in te r b e d d e d p u r p l i s h , impure l i m e s t o n e , and p a le
yellow-brown to w h i t e , f i n e g r a in e d dolom ite with some w hit e q u a r t z o s e
sa n d s to n e .
The Amsden Formation i s e a s i l y r e c o g n i z a b l e by i t s d i s t i n c ­
t i v e red c o l o r and i s t o p o g r a p h i c a l l y d i s p l a y e d as a low swale between
t h e two much more r e s i s t a n t Mission Canyon and Quadrant Formations.
Pennsylvanian
The Quadrant Formation g r a d a t i o n a l l y o v e r l i e s t h e Amsden Formation.
I t i s a w h i t e , l i g h t g r a y , and ye llo w t o p in k , medium t o t h i c k bedded,
f i n e - g r a i n e d q u a r t z o s e san ds to ne (McMannis, 1955).
Commonly, l i g h t g r a y ,
f i n e - g r a i n e d beds o f dolo mi te from 2 t o .4 f e e t in t h i c k n e s s occur in t h e
122
lower p o r t i o n o f t h e fo r m a ti o n .
The Quadrant Fo r m a t io n .s ta n d s high
t o p o g r a p h i c a l l y forming ledges and r i d g e s .
The Quadrant i s the lower­
most s t r a t a o f th e f l a t i r o n s t h a t help t o form t h e lower f a c e s o f t h e
mountain f r o n t .
R e r m ia n -T ri a s si c
Permian s t r a t a a r e r e p r e s e n t e d in t h e B rid ge r Range only by an
undefined u n i t o f l o c a l e x t e n t .
This i s b e li e v e d t o be an e r o s io n a l
remnant o f t h e Phosphoria Formation.
The u n i t c o n s i s t s o f c h e r t breccia;
a s l i g h t l y c a l c a r e o u s c h e r t and pebble conglomerate; a c h e r t bed; and a
yellow-brown t o p a l e g r a y i s h - p i n k , medium-grained, s l i g h t l y c a l c a r e o u s ,
i r r e g u l a r l y bedded s an ds to ne (McMannis, 1955).
S t r a t a o f T r i a s s i c Age a r e b e l i e v e d t o be a b s e n t in t h e Bridg er
Range.
Mesozoic
Jurassic
J u r a s s i c s t r a t a a r e r e p r e s e n t e d in t h e Bridg er Range by th e E l l i s
Group and t h e Morrison Formation.
th re e formations.
The E l l i s Group i s comprised of
These fo r m at i ons from o l d e s t t o . y o u n g e s t a r e th e
Sawtooth, R i e r d o n , and S w if t.
The Sawtooth i s o f Middle J u r a s s i c Age and i s e xtr e m el y v a r i a b l e
in t h i c k n e s s , ranging from as l i t t l e as 15 f e e t to as much as 145 f e e t ,
i t c o n s i s t s o f an upper red t o maroon s h a l e and a lower gra y s h a l e and
123
s h a l y s i l t s t o n e t h a t c o n t a i n s t h i n beds o f den se, dark gra y li m e s to ne.
The Sawtooth i s very f o s s i l i f e r o u s .
The Rierdon Formation, o f Middle t o Late J u r a s s i c Age, conformably
o v e r l i e s t h e Sawtooth.
A n o n - r e s i s t a n t upper lim e s to n e and c a lc a r e o u s
s h a l e and a b a s a l , m a s s iv e , o o l i t i c li m e s to n e make up t h e for ma ti on.
The o o l i t i c li m e s to n e i s a p e r s i s t e n t marker bed.
Like t h e Sawtooth,
th e Rierdon v a r i e s in t h i c k n e s s from l o c a l i t y to l o c a l i t y .
I t ranges
from O t o 114 f e e t .
The uppermost fo rm ati on o f th e E l l i s Group i s t h e S w i f t .
It
dis con fo rm ab ly o v e r l i e s t h e Rierdon and i s o f Late J u r a s s i c Age.
The
S w if t i s composed o f a ye ll o w is h -b ro w n , weat he rin g sa n d s to n e and sandy
lim e s to n e with a b a s a l , c h e r t - p e b b l e co nglo m e rati c zone.
common.
Fossils are
Thickness depends on l o c a l e , va ryi ng from 39 t o 100 f e e t .
The Morrison Formation o f Late J u r a s s i c Age conformably o v e r l i e s
the Swift.
I t i s non-marine in o r i g i n .
The Morrison c o n s i s t s of
v a r i e g a t e d red and green hackly s h a l e , s i l t s t o n e , and mudstone with
in te r b e d d e d yellow-brown t o p a l e brow nis h-o range, c a l c a r e o u s s i l t s t o n e
and sa n d s to n e .
In t h e lower p o r t i o n s a r e o f t e n found i r r e g u l a r l y
bedded lim e s to n e and lim e s to n e no dule s.
However, in t h e upper p o r t i o n s
o f t h e t h i c k e r s e c t i o n s , c a l c a r e o u s s h a l e i s common.
A maximum
t h i c k n e s s o f 444 f e e t f o r t h e Morrison Formation in t h e B ri d g e r Range
i s a t t a i n e d on t h e s o u t h e a s t f l a n k o f Sacagawea Peak.
I t i s 428 f e e t
124
a t F a i r y Lake (McMannis, 1955).
Many o f t h e open, p a r k - l i k e a r e a s on
t h e f l a n k o f t h e B rid ge rs a r e developed on t h e Morrison Formation.
Cretaceous
S t r a t a o f Cretaceous Age a r e r e p r e s e n t e d by t h e Kootenai Formation
and t h e Colorado and L i v in g s to n Groups. .
The Kootenai Formation o f E a rl y Cretaceous Age i s d i v i s i b l e i n t o
three u n its.
The lower u n i t i s a t h i c k , w h i t e , gray and p i n k - s t a i n e d ,
m a ss iv e , c r o s s - b e d d e d , c o a r s e - g r a i n e d , l o c a l l y co n g lo m e ra t ic san dst one .
The middle u n i t i s o f v a r i e g a t e d r e d , p u r p l e , and maroon s h a l e and
mudstone i n t e rb e d d e d with r u s t y - g r a y t o r e d - s t a i n e d , impure a r g i l l a c e o u s
s a n d s to n e .
The upper u n i t i s a medium t o t h i c k bedded, i r o n - s t a i n e d ,
pure q u a r t z o s e s an ds to ne c o n t a i n i n g t h i n , g r a y , s i l t y , f i s s i l e , and
p l a t y s h a l e beds.
The Kootenai in t h e n o r t h e r n B ri d g e rs measures 447
f e e t in t h i c k n e s s (McMannis, 1955).
Upper Cretaceous s t r a t a a r e r e p r e s e n t e d on t h e e a s t f l a n k o f t h e
B rid ge rs by t h e Colorado Group, t h e Eagle Formation, and t h e Liv in gsto n
Group.
The Colorado Group i s very p o o rl y exposed and very o f t e n has a
heavy ti m b e r co ve r.
T h e r e f o r e , exposures a r e d i f f i c u l t t o f i n d .
The
Colorado Group i s composed o f dark gray marine s h a l e , i r o n - s t a i n e d ,
g r a y - g r e e n , s i l t y s an ds to ne and s i l t s t o n e , and a few g r a y , t h i n bedded
salt-and-pepper sandstones.
W ri te rs from o t h e r a r e a s have div ide d t h e
C o l o r a d o Group i n t o f i v e fo r m a t i o n s ; t h e Thermopolis s h a l e with t h r e e
125
members, t h e Mowry Formation, t h e F r o n t i e r Formation, t h e Cody Shale
w it h t h r e e members, and t h e Te legraph Creek Formation.
In th e
s o u t h e a s t e r n p a r t o f t h e B rid ge r Range and eastward toward L i v i n g s t o n ,
Montana, Roberts (1972) d i f f e r e n t i a t e d and mapped t h e above named
fo r m a t i o n s .
Tonnsen (1975) mapped and measured s e c t i o n s o f th e F r o n t i e r
Formation in t h e B r i d g e r s .
For t h e most p a r t , exposures a r e d i f f i c u l t
t o l o c a t e , and t h e r e a r e many covered i n t e r v a l s (Tonnsen, o ra l com­
m u n i c a t i o n , 1975).
The Colorado Group a g g r e g a te s from 1200 t o 2400 f e e t
in t h i c k n e s s in t h e B r id g e r Range (McMannis, 1955).
The nex t you nge st Cretaceous fo rm ati on i s t h e Eagle.
I t consists
of l i g h t g r a y , t h i n bedded, s a l t - a n d - p e p p e r san dst one with int e rb e dded
carbonaceous s h a l e and t h i n coal seams.
T e r r e s t r i a l , b r a c k i s h , and
marine f o s s i l s a r e found in t h i s f o r m a ti o n .
Throughout much o f t h e
a r e a , t h e Eagle Formation forms l i n e a r to po g ra p h ic h ig h s .
The t h i c k n e s s
o f t h e Eagle ranges from 100 f e e t t o 600 f e e t .
Uppermost Cretaceous s t r a t a o f t h e B rid ge r Range a r e r e p r e s e n t e d
by t h e L i v in g s to n Group.
Much o f t h e lower e l e v a t i o n s in t h e a rea a r e
u n d e r l a i n by t h i s same Group.
McMannis (1955) r e c ogn iz e d f i v e mappable
u n i t s in t h e L i v in g st o n t h a t roughly co rrespond t o R o b e r t s ' (1963) f o u r
fo rm at io ns a t t h e type s e c t i o n o f t h e L iv in gst on Group n e a r Billman
Creek.
The ty pe s e c t i o n s o f t h e fo r m at i on s were measured from th e
abandoned town o f Cokedale , Montana, in Sec. 26, T 2 S, R 8 E and
n o r th e a s tw a rd t o Sec. 7 , T 2 S, R 9 E.
The uppermost u n i t o f McMannis
126
c or respo nds both in t h i c k n e s s and l i t h o l o g i c , composition t o Ro berts'
Lower Conglomerate and i s not c o n s id e r e d p a r t o f t h e Li vi n g st o n Group.
R a t h e r, i t i s th ought t o be t h e basal p a r t o f t h e Fo r t Union Formation
which o v e r l i e s t h e Li vin g st o n Group.
Mapping o f t h e L i v in g s to n Group rocks and e q u i v a l e n t s by va ri ou s
workers in t h e 1960's showed t h a t th e f o r m a t ! onal d i v i s i o n s o f R o b e r ts '
(1963) were only p a r t l y a p p l i c a b l e t o t h e a r e a e a s t o f t h e Bridger
Range.
The upper two f o r m a t i o n s , t h e Billman Creek and t h e Hoppers,
were d i s t i n g u i s h a b l e , whereas t h e lower two, t h e Cokedale and Miner
Creek, were no t ( Skipp and McGrew, 1972).
However, t h e s e a u th o rs were
a b l e t o d i s c e r n seven informal u n i t s e q u i v a l e n t t o t h e Cokedale and
Miner Creek Formations (Sk ipp , 1964; Skipp and McGrew, 1972).
These
u n i t s a r e ( I ) a lower s an ds to ne u n i t , (2) a welded t u f f u n i t , (3) t h e •
Parkman e q u i v a l e n t , (4) a middle san dst one u n i t , (5) a mudstone u n i t ,
(6) t h e Bearpaw e q u i v a l e n t , and (7) t h e Lennep e q u i v a l e n t .
The L i v in g s to n Group u n d e r l i e s a pprox im at el y t h e e n t i r e e a s t e r n
h a l f of th e map a r e a .
T h e r e f o r e , t h e fo ll ow in g review o f t h e forma-
t i o n a l u n i t s i s given f o r p o s s i b l e a p p l i c a t i o n in r e c o g n i z i n g the
r e l a t i o n s h i p s between t h e s e u n i t s and t h e i r r e s u l t i n g s u r f i c i a l cover.
The lower s an ds to ne u n i t forms low r i d g e s and somewhat s te e p s l o p e s
th ro ug ho ut t h e a r e a .
I t ranges from 500 t o 1000 f e e t in t h i c k n e s s .
The
u n i t i s a dark o l i v e - g r e e n , y e ll o w is h -b ro w n , v o l c a n i c s a n d s to n e , s i l t s t o n e , mudstone, p o r c e l a n i t e , and a l t e r e d c r y s t a l l i t h i c t u f f .
127
In te rb ed ded w ith t h e s e a r e pebble conglomerates and l i g n i t i c c o a l .
Calcareous c o n c r e t i o n s a r e p r e s e n t th ro ughou t t h e s an ds to ne (Skipp and
McGrew, 1972).
The welded t u f f u n i t ranges from 0 t o about 800 f e e t .
t h e a r e a , i t forms conspicuous r i d g e s .
Throughout
This u n i t i s composed o f t h r e e
f i n e - g r a i n e d , welded t u f f s h e e t s and a welded t u f f conglomerate which
i s i n t e rb e d d e d with v o l c a n i c s a n d s to n e , co nglomerate, mudstone, po rc e l a n i t e , and a l t e r e d y i t r i c t u f f .
The welded t u f f i s p a l e g re e n , o l i v e -
g r a y , g r a y i s h - r e d , and brown w hile t h e conglomerate u n i t i s o l i v e - g r e e n ,
dark g r e e n i s h - g r a y t o brown (Skipp and McGrew, 1972).
The Parkman e q u i v a l e n t i s 193 f e e t o f l i g h t g r a y , f i n e - g r a i n e d ,
c a l c a r e o u s and n o n c a lc a r e o u s , m o tt le d s a n d s to n e .
In te rb e d d e d with t h e
s an d s to n e a r e g r e e n , g r a y , and brown s i l i c e o u s mudstone, sa n d s to n e ,
c o n g l o m e r i t i c s a n d s t o n e , and a bed o f b i o t i t e - r i c h welded t u f f .
Leaf
and twig impr ess io ns a r e o f t e n found in t h e mudstones (Skipp and McGrew9
1972).
The middle san ds to ne u n i t c o n s t i t u t e s a r i d g e -f o rm in g s e r i e s o f
e p i c l a s t i c , v o l c a n i c san ds to ne in te r b e d d e d with v o l c a n i c conglomerate,
mudflow cong lom erat e, and v o l c a n i c s i l t s t o n e and mudstone.
ranges in t h i c k n e s s from 1000 f e e t t o 1,650 f e e t .
This u n i t
The sands tone s a r e
o l i v e - g r e e n and b l u e - g r e e n , f i n e - g r a i n e d t o c o n g lo m e r a t ic , m o t t l e d ,
c r o s s - b e d d e d , and c a l c a r e o u s .
Carbonaceous m a t t e r and z e o l i t e s a r e
common a c c e s s o r i e s (Skipp and McGrew, 1972).
128
The mudstone u n i t i s a v a l l e y forming u n i t about 600 f e e t to 1000
f e e t in t h i c k n e s s .
I t i s composed o f p o o rl y bedded v o l c a n i c mudstones.
Some o f t h e mudstones a r e s i l i c e o u s w hil e o t h e r s a r e v a r i o u s l y b e nto ­
n i t i c , sandy, and c a l c a r e o u s .
An e p i c l a s t i c , v o l c a n i c s a n d s to n e , c a l ­
careous in s p o t s , f i n e - g r a i n e d t o c onglo me rat ic i s in te r b e d d e d with t h e
mudstone.
Also found in t h e upper p a r t a r e t h i n beds o f b e n t o n i t e and
v itric tuff.
F r e s h -w a te r m o ll u s k s , wood and din o s au r bones have been
found in t h i s u n i t .
I t i s , however, c o n s id e r e d as a s e p a r a t e u n i t .
The name Bearpaw, a p p l i e d to t h e u n i t , i s taken from t h e e q u i v a l e n t
rocks to t h e n o r th and e a s t known as t h e Bearpaw S h a le .
The Bearpaw
e q u i v a l e n t c o n s i s t s o f p o o rl y exposed, gra y -g re e n mudstone with some
t h i n s an ds to ne and b e n t o n i t e beds (Skipp and McGrew, 1972).
The f i n a l informal u n i t o f t h e Cokedale-Miner Creek i n t e r v a l i s
th e Lennep e q u i v a l e n t .
As with t h e Bearpaw e q u i v a l e n t , t h e Lennep
e q u i v a l e n t comprises th o s e rocks t h a t a r e c o r r e l a t a b l e t o t h e Lennep
Sandstone t o t h e n o r th and e a s t .
I t v a r i e s from 350 f e e t t o 500 f e e t
in t h i c k n e s s and i s made up o f m a ri ne , b r a c k i s h , and non-marine ro c k s .
The rocks a r e s a n d s t o n e , cong lom erat e, mudstone, and a l t e r e d v i t r i c
tuff.
The san ds to ne s a r e c a l c a r e o u s , p o o rl y bedded t o c ro s s- b e d d e d ,
and fine-grained t o c on g lo m e ra t ic .
The conglomerate i s made up of
e p l i c l a s t i c , v o l c a n i c pebb les and c o b b l e s , some pebbles c o n s i s t i n g o f
P a le o z o i c l i m e s t o n e , and q u a r t z i t e .
F r es h -w a te r m o ll u s k s , a few burrows
1.29
o f marine o r i g i n , and some d i n o s a u r bones have been found in t h i s u n i t
(Skipp and McGrew, 1972).
Conformably o v e r l y i n g t h e Cokedale-Miner Creek i n t e r v a l in th e
Billman Creek Formation.
The for mati on t h i c k n e s s ranges from 2,500
f e e t t o 3,500 f e e t ( e s t i m a t e by Skipp and McGrew, 1972) w ith a lower
u n i t o f v a l l e y forming beds and an upper u n i t of r i d g e forming beds.
In g e n e r a l , t h e for matio n i s composed o f g r a y , g r e e n , and r e d calcareous
mudstone, in te r b e d d e d san ds to ne and s i l t s t o n e , and li m e s to n e nodules.
Conspicuous r i d g e s a r e formed in t h i s for mation by l e n s e s o f pebble and
cobble co nglom erate.
The pebbles and cobb les a r e o f a n d e s i t i c and
r h y o d a c i t i c v o l c a n i c ro c k s .
Included a r e some pebbles o f Paleo zoi c
lim e s to n e and q u a r t z i t e (Skipp and McGrew, 1972).
Dinasaur bones and mollusks a r e common in t h e lower p a r t s of t h e
fo r m a t i o n , w hi le f r e s h - w a t e r g a st r o p o d s and d i n o s a u r bones a r e common
in th e upper p a r t s (Skipp and McGrew, 1972).
The Hoppers Formation g r a d a t i o n a l Iy o v e r l i e s t h e Billman Creek
Formation.
The fo r m at io n i s f u l l y 1500 f e e t t h i c k an d, in some a r e a s ,
as much as 2400 f e e t .
The for mati on i s comprised o f e p i c l a s t i c , v o l ­
c a n ic s an ds to ne and s i l t s t o n e t h a t i s g r e e n i s h - g r a y , g r a y , p u r p l e - g r a y ,
c a l c a r e o u s , f i n e - g r a i n e d , and c ro s s - b e d d e d .
S c a t t e r e d thro ughou t t h e
u n i t a r e small p e b b l e , v o l c a n i c conglomerates and c l a y pebble conglo­
merates.
Also found in t h e for mati on a r e some w e ll- ro und e d g n e is s and
130
and q u a r t z pebb les in t h e lower 150 f e e t o f th e s e c t i o n (Skipp and
McGrew, 1972).
F o s s i l s found in t h i s for mation i n c l u d e p l a n t m a t e r i a l and wood
fragments as im pr ess io ns and a few f r e s h - w a t e r moll u s k s (Skipp and
McGrew, 1972).
Cenozoic
Tertiary
Rocks o f T e r t i a r y Age a r e r e p r e s e n t e d by t h e r e l a t i v e l y u n c o n s o l i ­
d a te d u n i t s o f t h e Bozeman Group.
T e r t i a r y d e p o s i t s o f t h e Bridger
Range crop o u t in only a few l o c a l i t i e s on t h e west s i d e o f t h e Range.
They a r e th o u g h t t o be t h e upper s t r a t i g r a p h i c e q u i v a l e n t s o f the
T e r t i a r y beds found in t h e c e n t r a l and west ern p a r t s o f t h e G a l l a t i n
V a lle y.
Robinson (1963) f o r m a l l y d i v i d e d t h e T e r t i a r y s e c t i o n in t h e
Three Forks Quadrangle i n t o t h e M il l i g a n Creek Formation, t h e Climbing
Arrow Formation, and t h e Dunbar Creek Formation.
Bozeman Group.
These were named t h e
They a r e a s s i g n e d ages o f Eocene, Late (and Middle?)
Eocene, and E a rl y Oligocene and Oligocene r e s p e c t i v e l y (Robinson, 1961
and 1963).
S t r a t a t o t h e e a s t o f t h e Three Forks Quadrangle a r e pro ­
g r e s s i v e l y younger.
The T e r t i a r y s t r a t a f l a n k i n g t h e w e st s i d e of t h e
Bri dgers a r e b e l i e v e d t o be Late Miocene and Pl i o ce n e in age (Robinson,
1961 and 1963).
S i m i l a r l y , Hack et t e t a l .(1960) d i v i d e d t h e T e r t i a r y
Bozeman Group i n t o t h r e e informal u n i t s .
The lowermost was recog nize d
131
only in t h e sub s u r f a c e , but t h e remaining two have s u r f a c e o u tc r o p s .
The upper two u n i t s were rec og ni ze d along t h e west s i d e o f t h e B rid ge rs
McMannis (1955) acknowledged s t r a t a o f T e r t i a r y Age which he roughly
d i v i d e d i n t o two u n i t s c o rr e sp ond in g w it h T e r t i a r y u n i t s mapped ne ar
Menard, Montana, by Klemme (1949).
These u n i t s correspo nd t o t h e upper
two u n i t s o f Ha ckett e t a l . (1960).
The lowermost o f t h e ou tc ro ppin g u n i t s c o n s i s t s o f well s t r a t i f i e d
v o l c a n i c a s h , t u f f a c e o u s marl and s i l t s t o n e , t u f f a c e o u s s a n d s to n e , some
beds o f led ge-fo rm in g lim e s to n e and, in some p l a c e s , cong lome rate s.
Some g a s t r a p o d s and o s t r a c o d s have been found in t h i s u n i t ( Hackett e t
a l . , 1960).
The upper u n i t i s composed o f p o o rl y s o r t e d , b u f f t o t a n , c a l ­
c a r e o u s , t u f f a c e o u s s i l t s t o n e , c l a y s t o n e , sand st one and conglomerate.
Also c o n ta i n e d w i t h i n t h i s u n i t a r e a few beds o f p u r e , gray a s h , and
small l e n s e s o f marl and l im e s to n e .
No f o s s i l s have been recovered
from t h e u n i t (Ha cke tt e t a l . , 1960).
Quaternary
Quater nary d e p o s i t s a r e found f a i r l y e x t e n s i v e l y developed on both
s i d e s o f t h e Range and c o n s i s t o f g ra ve l f a n s , r i v e r g r a v e l s and muds,
t a l u s , t e r r a c e g r a v e l s , morainal and outwash d e p o s i t s , mass wasting
d e p o s i t s , and avalanche d e b r i s .
This r e p o r t d e a l s o nly w it h th os e
Qua terna ry d e p o s i t s found on p a r t o f t h e n o r t h e a s t e r n f l a n k of th e
132
B ri d g e r Range,
Discu ssi on o f t h e s e d e p o s i t s i s given in t h e main t e x t
in t h e s e c t i o n on S u r f i c i a l Geology.
Plate I
R6 E . R7E
S U R F lC lA L GEOLOGY OF A PART
OF TH E N O R T H E A S T F L A N K OF
TH E BRIDGER RANGE, MONTANA
H ardscrabble Peak
9561
\
L EGEND
Qal
Qo~
Alluvium — O u t w a s h
It'S
Colluvium
Residuum
QUATERNARY
QC
Talus
Qm 4
P i n e d a l e III
Qm3
P i n e d a l e Il
Qm2
Qml
Pinedale I
Bull L a k e
Qrg
R o c k G lacier
Qm
U n d if f er e n tia te d M o r a i n e
M a ss W a s t i n g
wI
CRETACEOUS
K o o te n a i, C o l o r a d o , an d Eagle F o r m a t i o n s , a n d
the Livingson G r o u p
JURASSiC
—E ll i s Group a n d M o r r i s o n F o r m a t i o n
CARBONIFEROUS
M ad iso n a n d Big S n o w y G r o u p s , A m sd e n
Q u a d r a n t F o r m a ti o n s
DEVONIAN
Maywood, T h r e e F o rk s, a nd
Formations
CAMBRIAN
-Cu
Brackett
Creek
and
Sappington
F l a t h e a d , Wolsey, M e a g h e r , P a r k , P i l g r i m , a n d
Snow y R a n g e F o r m a t i o n s
- G e o lo g ic C o n t a c t s , D a s h e d W h e r e
I n ferre d
Stream s
6800
C o n t o u r s , Interval 4 0 0 f e e t
-f
Section C o rn e rs
-r
S U R F l C I AL GEOLOGY BY V E R N E K. S C H R U N K
Submitted in partial fulfillment of the requirements for the Degree of Master of
in EorthSciences at Montana State University
Science
1976
R 6 E R7 E
t
o
b
I -
Mi l e s
LIBRARY
MONTANA STATE UNIVERSITY"
BOZEMAN
Nm
Seh 7(r
'
\
I
V >
Bedrock Geology adapted from McMannis(l955)
and Skipp and MoMannis(1971 ).
MONTANA STATE UNIVERSITY LIBRARIES